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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	May 11, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	SYBX
Entity Registrant Name	SYNLOGIC, INC.
Entity Central Index Key	0001527599
Current Fiscal Year End Date	--12-31
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		25,450,808

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 27,185	$ 58,440
Short-term marketable securities	98,618	28,585
Prepaid expenses and other current assets	2,057	1,564
Total current assets	127,860	88,589
Property and equipment, net	13,704	9,783
Restricted cash	1,097	1,097
Other assets		230
Total assets	142,661	99,699
Current liabilities:
Accounts payable	1,336	2,679
Accrued expenses	4,237	4,823
Deferred revenue	434	444
Deferred rent	621	656
Capital lease obligations	382	425
Total current liabilities	7,010	9,027
Long-term liabilities:
Deferred revenue, net of current portion		668
Deferred rent, net of current portion	6,483	4,500
Capital lease obligations, net of current portion	401	466
Total long-term liabilities	6,884	5,634
Commitments and contingencies (Note 14)
Stockholders' Equity
Preferred stock, $0.001 par value 5,000,000 shares authorized, none issued and outstanding as of March 31, 2018 and December 31, 2017
Common stock, $0.001 par value 250,000,000 shares authorized as of March 31, 2018 and December 31, 2017. 22,172,117 and 16,272,617 shares issued and outstanding as of March 31, 2018 and December 31, 2017, respectively.	22	16
Additional paid-in capital	211,351	156,685
Accumulated other comprehensive loss	(111)	(9)
Accumulated deficit	(82,495)	(71,654)
Total stockholders' equity	128,767	85,038
Total liabilities and stockholders' equity	$ 142,661	$ 99,699

Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, Shares Authorized	5,000,000	5,000,000
Preferred stock, Shares Issued	0	0
Preferred stock, Shares Outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, authorized	250,000,000	250,000,000
Common stock, Issued	22,172,117	16,272,617
Common stock, outstanding	22,172,117	16,272,617

Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenue	$ 354	$ 111
Operating expenses:
Research and development	8,361	5,118
General and administrative	3,629	2,367
Total operating expenses	11,990	7,485
Loss from operations	(11,636)	(7,374)
Other income (expense):
Interest and investment income	486	16
Interest expense	(14)	(8)
Other expense	(1)	(2)
Other income (expense), net	471	6
Net loss	$ (11,165)	$ (7,368)
Net loss per share attributable to common shareholders - basic and diluted	$ (0.55)
Weighted-average common shares used in computing net loss per share attributable to common shareholders - basic and diluted	20,145,881
Net loss per unit attributable to common unit holders - basic and diluted		$ (4.49)
Weighted-average common units used in computing net loss per unit attributable to common unit holders - basic and diluted		1,640,367
Comprehensive loss:
Net loss	$ (11,165)	$ (7,368)
Net unrealized losses on marketable securities	(102)
Comprehensive loss	$ (11,267)	$ (7,368)

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (11,165)	$ (7,368)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	681	222
Loss on disposal of property and equipment	2
Equity-based compensation expense	919	130
Accretion/amortization of investment securities	(171)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(493)	320
Accounts payable and accrued expenses	(2,975)	(41)
Deferred revenue	(354)	(111)
Deferred rent	295	(63)
Other assets	230	7
Net cash used in operating activities	(13,031)	(6,904)
Cash flows from investing activities:
Purchases of marketable securities	(74,964)
Proceeds from maturity of marketable securities	5,000
Purchases of property and equipment	(1,904)	(125)
Net cash used in investing activities	(71,868)	(125)
Cash flows from financing activities:
Payments on capital lease obligations	(108)	(60)
Proceeds from sale of common stock, net of issuance costs	53,752
Proceeds from sale of preferred units, net of issuance costs		26,649
Net cash provided by financing activities	53,644	26,589
Net (decrease) increase in cash, cash equivalents and restricted cash	(31,255)	19,560
Cash, cash equivalents and restricted cash at beginning of period	59,537	14,636
Cash, cash equivalents and restricted cash at end of period	28,282	34,196
Supplemental disclosure of non-cash investing activities:
Landlord funded allowance for tenant improvements	1,653
Property and equipment purchases included in accounts payable and accrued expenses	1,047	18
Supplemental disclosure of non-cash financing activities:
Cash paid for interest	$ 14	$ 8

Nature of Business

3 Months Ended

Mar. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of Business

(1)	Nature of Business

Organization

Synlogic, Inc., together with its wholly owned and consolidated subsidiaries (“Synlogic” or the “Company”), is a clinical-stage biopharmaceutical company focused on advancing its drug discovery and development platform for Synthetic Biotic™ medicines, which are designed using synthetic biology to genetically reprogram beneficial microbes to treat metabolic and inflammatory diseases and cancer. Synthetic Biotic medicines are generated from Synlogic’s proprietary drug discovery and development platform applying the principles and tools of synthetic biology to engineer beneficial probiotic bacteria to perform or deliver critical therapeutic functions. As living medicines, Synthetic Biotic medicines can be designed to sense a local disease context within a patient’s body and to respond by metabolizing a toxic substance, by compensating for missing or damaged metabolic pathways in patients, or by delivering combinations of therapeutic factors. Synlogic’s goal is to lead in the discovery and development of Synthetic Biotic therapies as living medicines capable of robust and precise pathway complementation and delivery of therapeutic benefit.

Synlogic, Inc. (“Private Synlogic” when referred to prior to the Merger (as defined below)) was founded and began operations on March 14, 2014, as TMC Therapeutics, Inc., located in Cambridge, Massachusetts. On July 15, 2014, TMC Therapeutics, Inc. changed its name to Synlogic, Inc. On July 2, 2015, the common and preferred stockholders of Private Synlogic executed the Synlogic, LLC Contribution Agreement (the “Contribution Agreement”), pursuant to which such common and preferred stockholders contributed such stockholders’ equity interests in Private Synlogic in exchange for common and preferred units in a newly formed parent company named Synlogic, LLC. In addition, Synlogic IBDCo, Inc. (“IBDCo”) was formed as a subsidiary of Synlogic, LLC (the “2015 Reorganization”). In conjunction with the 2015 Reorganization, Private Synlogic entered into a license, option and merger agreement with AbbVie S.à.r.l. (“AbbVie”), for the development of treatments for inflammatory bowel disease (“IBD”). See Note 12, AbbVie Collaboration Agreement.

In May 2017, Private Synlogic completed a reorganization (the “2017 Reorganization”) pursuant to which Synlogic, LLC merged with and into Private Synlogic, with Private Synlogic continuing as the surviving corporation.

On August 28, 2017, Synlogic, Inc., formerly known as Mirna Therapeutics, Inc. (NASDAQ: MIRN) (“Mirna”), completed its business combination with Private Synlogic pursuant to the Agreement and Plan of Merger and Reorganization, dated as of May 15, 2017, by and among Mirna, Meerkat Merger Sub, Inc. (“Merger Sub”), and Private Synlogic (the “Merger Agreement”), pursuant to which Merger Sub merged with and into Private Synlogic, with Private Synlogic surviving as a wholly owned subsidiary of Mirna (the “Merger”). Immediately after completion of the Merger, Mirna changed its name to “Synlogic, Inc.” (NASDAQ: SYBX). See Note 3, Merger with Mirna Therapeutics.

The Company operates in one operating segment: the discovery and development of Synthetic Biotic medicines. The Company’s chief executive officer, as chief operating decision maker, manages and allocates resources to the operations of the Company on a total company basis. Since incorporation, the Company has devoted substantially all of its efforts to the research and development of its product candidates.

Risks and Uncertainties

At March 31, 2018, the Company had $125.8 million in cash, cash equivalents, and marketable securities, $1.1 million of restricted cash and an accumulated deficit of $82.5 million. Since its inception through March 31, 2018, the Company has primarily financed its operations through the issuance of preferred stock and units, the sale of its common stock, the AbbVie collaboration, and cash received in the Merger. In the absence of positive cash flows from operations, the Company is highly dependent on its ability to find additional sources of funding in the form of debt or equity financing. In January 2018, the Company sold a total of 5,899,500 shares of its common stock in a firm commitment, underwritten public offering at a price of $9.75 per share. The Company received $53.8 million in net proceeds from this offering, after underwriting discounts and commission and other offering expenses. Management believes that the Company has sufficient cash to fund its operations through at least twelve months from the issuance of these financial statements.

As an early-stage company, the Company is subject to a number of risks common to other life science companies, including, but not limited to, raising additional capital, development by its competitors of new technological innovations, risk of failure in preclinical and clinical studies, safety and efficacy of its product candidates in clinical trials, the risk of relying on external parties such as contract research organizations (“CROs”) and contract manufacturing organizations (“CMOs”), the regulatory approval process, market acceptance of the Company’s products once approved, lack of marketing and sales history, dependence on key personnel and protection of proprietary technology. The Company’s therapeutic programs are currently pre-commercial, spanning discovery through early development and will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization of any product candidates. These efforts require significant amounts of additional capital, adequate personnel, infrastructure, and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company may never achieve profitability, and unless and until it does, it will continue to need to raise additional capital or obtain financing from other sources, such as strategic collaborations or partnerships.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

(2)	Summary of Significant Accounting Policies

The significant accounting policies described in the Company’s audited financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission (“SEC”) on March 20, 2018, have had no material changes during the three months ended March 31, 2018, other than our adoption of ASC 606 (as defined below) which is discussed in detail in this note.

Basis of Presentation

The accompanying consolidated financial statements and the related disclosures as of March 31, 2018 and for the three months ended March 31, 2018 and 2017 are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) and the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. These interim consolidated financial statements should be read in conjunction with the Company’s 2017 and 2016 audited consolidated financial statements and notes included in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018. The December 31, 2017 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures including notes required by GAAP for complete financial statements. In the opinion of management, the unaudited interim consolidated financial statements reflect all adjustments, consisting of normal and recurring adjustments, necessary for the fair presentation of the Company’s financial position and results of operations for the three months ended March 31, 2018 and 2017. The results of operations for the interim periods are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or any other interim period or future year or period.

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of Synlogic and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.

Recently Adopted Accounting Pronouncements

Revenue Recognition

In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC 605, Revenue Recognition, or ASC 605, and creates ASC 606, Revenue from Contracts with Customers. In 2015 and 2016, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients.

Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers, or ASC 606, using the modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC 605. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five-step analysis: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step analysis to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.

The Company has entered into collaboration agreements for research and development services, under which the Company licenses certain rights to its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non-refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. Any variable consideration is constrained, and therefore the cumulative revenue associated with the consideration is not recognized, until it is deemed not be at significant risk of reversal.

In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under each of its agreements for which the collaboration partner is also a customer, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; and c) the contract term and pattern of satisfaction of the performance obligations under step (v) above. The Company uses significant judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to the goods and services the Company expects to provide. The Company uses estimates to determine the timing of satisfaction of performance obligations, which may include the use of full time employees (“FTE”) as a measure of satisfaction of performance obligations.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current deferred revenue. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion.

Licenses of Intellectual Property

In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promises, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Research and Development Services

If an arrangement is determined to contain a promise or obligation for the company to perform research and development services, the Company must determine whether these services are distinct other promises in the arrangement. In assessing whether the services are distinct from the other promises, the Company considers the capabilities of the customer to perform these same services. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For research and development services that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Customer Options

If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, that is, the option to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on an alternative approach when the goods or services are both (i) similar to the original goods and services in the contract and (ii) provided in accordance with the terms of the original contract. Under this alternative, the Company allocates the total amount of consideration expected to be received from the customer to the total goods or services expected to be provided to the customer. Amounts allocated to a material right are not recognized as revenue until the option is exercised and the performance obligation is satisfied.

Milestone Payments

At the inception of each arrangement that includes milestone payments, the Company evaluates whether a significant reversal of cumulative revenue provided in conjunction with achieving the milestones is probable and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. For other milestones, the Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal of cumulative revenue would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment.

Royalties

For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements.

Contract costs

The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer.

For additional discussion of accounting for collaboration revenues, see Note 12, AbbVie Collaboration Agreement.

Impact of Adoption

As a result of adopting ASC 606 on January 1, 2018, the Company has recorded a cumulative-effect decrease to opening accumulated deficit of $0.3 million as of January 1, 2018 and a corresponding decrease to deferred revenue. Total revenue recorded in the three months ended March 31, 2018 under ASC 606 was $0.4 million, as compared to $0.1 million that would have been recorded under ASC 605. Deferred revenue as of March 31, 2018 was $0.4 million under ASC 606, as compared to a balance of $1.0 million which would have resulted under ASC 605.

The most significant changes relate to the Company’s revenue recognition pattern for the AbbVie Collaboration Agreement and the accounting for milestone payments.

Under ASC 605, the Company was recognizing the revenue allocated to each unit of accounting on a straight‑line basis over the period the Company is expected to complete its obligations. Under ASC 606, the Company is recognizing the revenue allocated to each performance obligation measuring progress using an input method over the period the Company is expected to complete each performance obligation.

Under ASC 605, the Company recognized revenue related to milestone payments as the milestone was achieved, using the milestone method. Under ASC 606, the Company determined that the milestones at the beginning of certain research and development phases represent a 90-day contract with daily customer renewal options for the Company’s continued research and development services. As a result, revenue from these milestones is recognized over a performance obligation consisting of the next phase of research and development services. For further discussion of the adoption of this standard, see Note 12, AbbVie Collaboration Agreement.

Stock Compensation

In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting. The new standard is intended to reduce the diversity in practice and cost and complexity when applying the guidance in Topic 718 to a change to the terms or conditions of a share-based payment award. The new standard will be effective for annual reporting periods and interim periods within those annual periods, beginning after December 15, 2017. The amendments in this update will be applied prospectively to an award modified on or after the adoption date. The Company adopted this standard as of January 1, 2018 and it did not have a material impact on the Company’s financial position or results of operations.

Recently Issued Accounting Pronouncements

The recently issued accounting pronouncements described in the Company’s consolidated financial statements as of and for the year ended December 31, 2017, and the notes thereto included in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018, have had no material changes during the three months ended March 31, 2018, except as described below.

In February 2016, the FASB issued ASU 2016-02 – Leases (Topic 842), which replaces the existing accounting guidance for leases. This standard requires entities that lease assets to recognize the assets and liabilities for the rights and obligations created by those leases on the balance sheet. The standard is effective for fiscal years and the interim periods within those fiscal years beginning after December 15, 2018. The guidance is required to be applied by the modified retrospective transition approach and early adoption is permitted. While the Company is currently assessing the impact that adoption of this guidance will have on its financial statements and footnote disclosures, it anticipates it will result in an increase in assets and liabilities.

In February 2018, the FASB issued ASU 2018-02 – Income Statement – Reporting Comprehensive Income (Topic 220), which provides amended guidance on income tax accounting. The amended guidance permits the reclassification of the income tax effect on amounts recorded within other comprehensive income impacted by the Tax Cuts and Jobs Act into retained earnings. The amended guidance is effective for periods ending after December 15, 2018 and applies only to those amounts remaining in Other Comprehensive Income at the date of enactment of the Act. The amended guidance may be adopted on either a retrospective basis or at the beginning of the period of adoption. The Company is assessing the potential impact of the amended standard.

In March 2018, the FASB issued Accounting Standards Update No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 (“ASU 2018-05”). The standard amends Accounting Standards Codification 740, Income Taxes (“ASC 740”), to provide guidance on accounting for the tax effects of the Tax Act pursuant to Staff Accounting Bulletin No. 118. The Company is currently evaluating the new guidance included in ASU 2018-05, but does not expect it to have a material impact on its consolidated financial statements.

Merger with Mirna Therapeutics

3 Months Ended

Mar. 31, 2018

Business Combinations [Abstract]

Merger with Mirna Therapeutics

(3)	Merger with Mirna Therapeutics

On August 28, 2017, Synlogic, Inc., formerly known as Mirna, completed its business combination with Private Synlogic in accordance with the terms of the Agreement and Plan of Merger and Reorganization, dated as of May 15, 2017, by and among Mirna, Meerkat Merger Sub, Inc. (“Merger Sub”), and Private Synlogic (the “Merger Agreement”), pursuant to which Merger Sub merged with and into Private Synlogic, with Private Synlogic surviving as a wholly owned subsidiary of Mirna. On August 25, 2017, in connection with, and prior to the completion of, the Merger, Mirna effected a 1:7 reverse stock split of its common stock (the “Reverse Stock Split”), and on August 28, 2017, immediately after completion of the Merger, Mirna changed its name to “Synlogic, Inc.” (NASDAQ: SYBX). Pursuant to the terms of the Merger Agreement and after giving effect to the Reverse Stock Split, at the effective time of the Merger (the “Effective Time”), each outstanding share of Private Synlogic capital stock was converted into the right to receive approximately 0.5532 shares of Mirna common stock (the “Exchange Ratio”). In addition, at the Effective Time, Mirna assumed all outstanding options to purchase shares of Private Synlogic common stock, which were exchanged for options to purchase shares of Mirna common stock, in each case appropriately adjusted based on the Exchange Ratio. Mirna also assumed the 2017 Plan. Immediately after the Merger, there were 16,282,496 shares of common stock outstanding.

For accounting purposes, Private Synlogic is considered to have acquired Mirna in the Merger. Private Synlogic was determined to be the accounting acquirer based upon the terms of the Merger and other factors including: (i) Private Synlogic stockholders owned approximately 83% of the combined company immediately following the closing of the Merger, (ii) Private Synlogic directors held five of the seven board seats in the combined company, and (iii) Private Synlogic management held all key positions in the management of the combined company. The Merger was accounted for as an asset acquisition rather than a business combination because the assets acquired and liabilities assumed by the Company did not meet the definition of a business as defined by ASU 2017-01. The net assets acquired in connection with this transaction were recorded at their estimated acquisition date fair values as of August 28, 2017, the date the Merger was completed (the “Merger Closing Date”). For additional disclosure related to the Merger, see Note 3, Merger with Mirna, in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018.

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

(4)	Fair Value of Financial Instruments

The table below presents information about the Company’s assets that are measured at fair value on a recurring basis and indicate the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value, as described under Note 2, Summary of Significant Accounting Policies.

The Company’s investment portfolio includes many fixed income securities that do not always trade on a daily basis. As a result, the pricing services used by the Company applied other available information as applicable through processes such as benchmark yields, benchmarking of like securities, sector groupings and matrix pricing to prepare evaluations. In addition, model processes were used to assess interest rate impact and develop prepayment scenarios. These models take into consideration relevant credit information, perceived market movements, sector news and economic events. The inputs into these models may include benchmark yields, reported trades, broker-dealer quotes, issuer spreads and other relevant data.

At March 31, 2018 and December 31, 2017, the Company has classified assets measured at fair value on a recurring basis as follows (in thousands):

	Fair Value Measurements at Reporting Date Using
	March, 31		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	15,816	$	15,816	$	—	$	—
Corporate debt securities (included in cash and cash equivalents)		1,246		—		1,246		—
Corporate debt securities (included in short- term investments)		98,618		—		98,618		—
Total	$	115,680	$	15,816	$	99,864	$	—

	Fair Value Measurements at Reporting Date Using
	December 31,		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	21,301	$	21,301	$	—	$	—
Corporate debt securities (included in cash and cash equivalents)		11,405		—		11,405		—
Corporate debt securities (included in short- term investments)		28,585		—		28,585		—
Total	$	61,291	$	21,301	$	39,990	$	—

Cash equivalents, prepaid expenses and other current assets, accounts payable and accrued expenses at March 31, 2018 and December 31, 2017 are carried at amounts that approximate fair value due to their short-term maturities. Capital lease obligations at March 31, 2018 and December 31, 2017 approximate fair value as they bear interest at a rate approximating a market interest rate.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Available-for-Sale Investments

3 Months Ended

Mar. 31, 2018

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Investments

(5)	Available-for-Sale Investments

The following tables summarize the available-for-sale securities held at March 31, 2018 and December 31, 2017 (in thousands):

March 31, 2018

Amortized cost

Gross unrealized

gains

Gross unrealized

losses

Fair Value

Corporate debt securities

98,729

(118

)

98,618

Total

98,729

(118

)

98,618

December 31, 2017

Amortized cost

Gross unrealized

gains

Gross unrealized

losses

Fair Value

Corporate debt securities

28,593

)

28,585

Total

28,593

)

28,585

The contractual maturity of all securities held at March 31, 2018 was one year or less. There were 30 and seven investments in an unrealized loss position at March 31, 2018 and December 31, 2017, respectively, none of which had been in an unrealized loss position for more than twelve months. The aggregate fair value of the securities in an unrealized loss position at March 31, 2018 and December 31, 2017 was $86.3 million and $19.3 million, respectively. The Company reviews its investments for other-than-temporary impairment whenever the fair value of an investment is less than amortized cost and evidence indicates that an investment’s carrying amount is not recoverable within a reasonable period of time. To determine whether an impairment is other-than-temporary, the Company considers whether it has the ability and intent to hold the investment until a market price recovery and considers whether evidence indicating the cost of the investment is recoverable outweighs evidence to the contrary. The Company did not hold any securities with an other-than-temporary impairment at March 31, 2018.

Gross realized gains and losses on the sales of investments have not been material to the Company’s consolidated statement of operations.

Property and Equipment, Net

3 Months Ended

Mar. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

(6)	Property and Equipment, net

Property and equipment, net consists of the following (in thousands):

	March 31,			December 31,
	2018			2017
Laboratory equipment	$	3,936		$	2,999
Computer and office equipment		739			354
Furniture and fixtures		409			220
Leasehold improvements		8,504			2,308
Construction in progress		1,389			7,017
		14,977			12,898
Less accumulated depreciation		(1,273	)		(3,115	)
	$	13,704		$	9,783

At March 31, 2018 and December 31, 2017, leasehold improvements include $6.6 million and $1.3 million of lessor-paid tenant improvements for which the Company was deemed to be the accounting owner of the tenant improvements primarily because it was responsible for project cost overruns. Also, at December 31, 2017, construction in progress contained $5.0 million of lessor-paid tenant improvements that were placed in service in the first quarter of 2018, for which the Company was deemed to be the accounting owner primarily because it was responsible for project cost overruns.

The Company has entered into leases for certain laboratory equipment which were capital leases. The leases had either a present value of expected payments in excess of 90% of the fair value of the equipment or a bargain purchase option at the end of the lease. As such, as of March 31, 2018 and December 31, 2017 the Company had $1.3 million and $1.4 million, respectively, of assets under a capital lease, having accumulated depreciation of $0.2 million in both periods.

Accrued Expenses

3 Months Ended

Mar. 31, 2018

Payables And Accruals [Abstract]

Accrued Expenses

(7)	Accrued Expenses

Accrued expenses consists of the following (in thousands):

	March 31,		December 31,
	2018		2017
Payroll related	$	823	$	1,721
Professional fees		671		805
Research and development		1,028		2,027
Other		1,715		270
	$	4,237	$	4,823

Other accrued expenses include $1.3 million and $31,000 in partially-completed fixed assets recorded in construction in progress at March 31, 2018 and December 31, 2017, respectively.

Common Stock

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Common Stock

(8)	Common Stock

The Company’s common stock has the following characteristics:

•

The holders of shares of common stock are entitled to one vote for each share of common stock held at all meetings of stockholders.

•

The holders of shares of common stock are entitled to receive dividends, if and when, declared by the Company’s board of directors. Since inception, no cash dividends have been declared.

On January 26, 2018, the Company sold 5,130,000 shares of its common stock through a firm commitment, underwritten public offering at a price to the public of $9.75 per share. On January 31, 2018, the underwriters elected to exercise their option to purchase 769,500 additional shares of common stock at the public offering price, less underwriting discounts and commissions. As a result of the offering, including the exercise of the overallotment option, the Company received aggregate net proceeds of $53.8 million, after underwriting discounts and commissions and other offering expenses.

At March 31, 2018, the Company held repurchase or forfeiture rights relating to 360,527 shares of common stock. The repurchase rights are at a price equal to the initial purchase price by the founders of Private Synlogic, adjusted by the Merger Exchange Ratio. The repurchase right lapses over time and is exercisable should the founders cease providing services to the Company prior to the end of a four-year period which began in April or May 2014, as the case may be. The forfeiture right lapses over time and is triggered when a holder ceases providing services to the Company. As of March 31, 2018 and December 31, 2017, the Company has exercised its repurchase right as to 41,819 shares of common stock and 16,111 shares of common stock have been forfeited back to the Company.

Preferred Stock

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Preferred Stock

(9)	Preferred Stock

Preferred Stock of Synlogic, Inc.

The Company’s preferred stock may be issued from time to time in one or more series, with each such series to consist of such number of shares and to have such terms as adopted by the board of directors. Authority is given to the board of directors to determine and fix such voting powers, full or limited, or no voting powers, and such designations, preferences and relative participating, optional or other special rights, and qualifications, limitation or restrictions thereof, including without limitation, dividend rights, conversion rights, redemption privileges and liquidation preferences. At March 31, 2018 and December 31, 2017, no shares of preferred stock had been issued.

Preferred Stock of Private Synlogic

Prior to the Merger, Private Synlogic had contingently redeemable preferred stock and three series of convertible preferred stock. On the Merger Closing Date, Mirna issued shares of its common stock to holders of these shares, at an exchange rate of 0.5532 shares of common stock, after taking into account the Reverse Stock Split, in exchange for each share of preferred stock outstanding immediately prior to the Merger.

Pursuant to, and at the time of, the 2017 Reorganization, preferred stock was granted to all holders of preferred units. The Private Synlogic preferred stock had substantially similar rights and preferences as the preferred units, except that the preferred stock was convertible into common stock at the option of the holder, on a one-for-one basis, subject to an anti-dilution adjustment. Conversion of the preferred stock would have been automatically triggered upon a firm-commitment underwritten public offering or upon a supermajority preferred interest vote. For a description of the rights and preferences of the preferred stock of Private Synlogic, refer to the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018.

Preferred Units

3 Months Ended

Mar. 31, 2018

Equity [Abstract]

Preferred Units

(10)	Preferred Units

Prior to the 2017 Reorganization, the Company had one class of contingently redeemable preferred units and two classes of convertible preferred units.

Pursuant to the 2015 Reorganization, each share of the Company’s Series A Preferred Stock and Series A Contingently Redeemable Preferred Stock was exchanged for a like type and number of the Company’s Class A Preferred Units and Contingently Redeemable Class A Preferred Units, respectively. Refer to the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018, for detail on the rights and preferences of Private Synlogic’s Preferred Units.

Equity-based Compensation and Equity Incentive Plans

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity-based Compensation and Equity Incentive Plans

(11)	Equity‑based Compensation and Equity Incentive Plans

The Company is displaying all equity in its post-Merger amounts, as impacted by the Exchange Ratio.

Equity Plans

The Company has a number of equity plans, two of which are currently active.

The 2015 Equity Incentive Award Plan (the “2015 Plan”) was adopted by Mirna in 2015 and remains active after the Merger, now functioning as the primary equity plan for the Company. The 2015 Plan provides for the granting of a variety of stock‑based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, deferred stock awards, dividend equivalent awards, stock payment awards, performance awards and other stock‑based awards.. Pursuant to the evergreen provision of the 2015 Plan, which allows for an annual increase in the number of shares of common stock available for issuance, the Company added 813,630 shares to the 2015 Plan on January 1, 2018.

The 2017 Stock Incentive Plan (the “2017 Plan”) was adopted by Private Synlogic in 2017 at the time of the 2017 Reorganization and assumed by the Company during the Merger. The 2017 Plan provides for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards.

As of March 31, 2018, there were 425,133 shares available for future grant under the Company’s two active equity incentive plans, the 2017 Plan and the 2015 Plan.

For a full description of the Company’s equity plans, refer to Note 12, Equity-based Compensation and Equity Plans in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018.

Stock Options

There were no options issued during the three months ended March 31, 2017. The following table summarizes stock option activity during the three months ended March 31, 2018, as adjusted for the Exchange Ratio under the 2015 Plan and the 2017 Plan.

	Stock options outstanding
						Weighted
				Weighted		average		Aggregate
				average		remaining		Intrinsic
	Number of			exercise		contractual		value^(a)
	options			price		term		(in thousands)
Outstanding at December 31, 2017		1,267,221		$	13.62		9.6	$	—
Granted		479,096			10.01		9.9		—
Exercised		—			—				—
Forfeited		(4,790	)		13.53		9.2
Outstanding at March 31, 2018		1,741,527			12.63		9.5	$	—

Vested or expected to vest at March 31, 2018		1,741,527			12.63		9.5		—

Exercisable at March 31, 2018		198,944			13.62		9.1	$	—

(a)

The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at March 31, 2018 and December 31, 2017. No options were in the money at March 31, 2018 or December 31, 2017.

As of March 31, 2018, there was $11.4 million of unrecognized share-based compensation related to employees for unvested stock option grants which is expected to be recognized over a weighted average period of 3.1 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. In addition, there was $0.1 million of unrecognized share-based compensation, related to unvested stock option grants to non-employees which is expected to be recognized over a weighted average period of 0.7 years. The amount of equity-based compensation expense related to non-employees that will ultimately be recorded will depend on the remeasurement of the outstanding awards through their vesting date.

Restricted Common Stock

The following table shows restricted stock activity during the three months ended March 31, 2018:

	Restricted stock awards
				Grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2017		375,479		$	13.55
Granted		—			—
Vested		(55,313	)		13.53
Forfeited		—			—
Unvested at March 31, 2018		320,166		$	13.56

As of March 31, 2018, there was $0.5 million of unrecognized share-based compensation related to restricted stock awards granted to employees, which is expected to be recognized over a weighted average period of 1.9 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur. In addition, there was $33,000 of unrecognized share-based compensation, related to unvested restricted stock awards granted to non-employees which is expected to be recognized over a weighted average period of 0.4 years.

Incentive Units

Prior to the 2017 Reorganization described above, incentive units were issued by Synlogic, LLC under the 2015 LLC Plan.

No incentive units were issued during the three months ended March 31, 2017. In May 2017, all incentive units were cancelled pursuant to the 2017 Reorganization and reissued as restricted common stock. As a result, there was no unrecognized compensation expense related to incentive units as of March 31, 2018.

Restricted Common Units

Prior to the 2017 Reorganization described above, restricted common unit awards were issued by Synlogic, LLC under the 2015 LLC Plan. No restricted common unit awards were issued during the three months ended March 31, 2017. In May 2017, the restricted common unit award was cancelled pursuant to the 2017 Reorganization and reissued as restricted common stock. As a result, there was no unrecognized compensation expense related to unvested restricted common units as of March 31, 2018.

Equity Compensation

The Company recorded total equity-based compensation expense of $0.9 million and $0.1 million during the three months ended March 31, 2018 and 2017, respectively. Equity compensation during the three months ended March 31, 2018 is derived from stock options and restricted stock awards, while equity compensation during the three months ended March 31, 2017 was derived from incentive units and restricted common units.

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the three months ended March 31, 2018 and 2017 (in thousands):

	Three months ended March 31,
	2018		2017
Research and development	$	377	$	59
General and administrative		542		71
	$	919	$	130

The following table summarizes equity‑based compensation expense by type of award for the three months ended March 31, 2018 and 2017 (in thousands):

	Three months ended March 31,
	2018		2017
Stock options	$	773	$	—
Restricted stock awards		146		—
Incentive units		—	$	96
Restricted common units		—		34
	$	919	$	130

AbbVie Collaboration Agreement

3 Months Ended

Mar. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

AbbVie Collaboration Agreement

(12)	AbbVie Collaboration Agreement

In July 2015, the Company entered into the AbbVie Agreement under which the Company granted AbbVie an exclusive option to purchase IBDCo and, in exchange, agreed to collaborate in researching and developing an Investigatory New Drug (“IND”) candidate for the treatment of IBD. The AbbVie Agreement sets forth the Company’s and AbbVie’s respective obligations for development and delivery of an IND candidate package using reasonable commercial efforts.

In exchange for the exclusive option to acquire IBDCo, initial research and development services for drug discovery and pre-clinical development, and participation on the joint research committee (“JRC”), AbbVie agreed to pay IBDCo an upfront, nonrefundable cash payment of $2.0 million, which IBDCo received in December 2015. AbbVie also agreed to pay IBDCo up to $16.5 million in milestone payments associated with specified research and pre-clinical events, which were determined to represent customer options for accounting purposes, as well as an option exercise fee upon the execution of their option to buy IBDCo and other royalty and milestone payments. The upfront cash payment and any payments for option fees and royalties are non-refundable, non-creditable and not subject to set-off.

The research and development will be performed by the Company over an estimated period of 54 months according to four phases of research defined in the research plan. The Company is eligible to receive payments from AbbVie upon the election to continue the research and development at the achievement of certain milestone events. The JRC will make a determination as to the continuation of the collaboration at the achievement of research and pre-clinical milestones, except for the final milestone, which AbbVie has the discretion to determine achievement without the approval of the JRC. If the parties make the determination to continue on with the AbbVie Agreement upon achievement of each milestone event, then AbbVie will pay the consideration associated with that milestone and the collaboration will continue through the remaining term of the option to purchase IBDCo, which is 54 months. However, AbbVie has the right to terminate the contract at any time with 90-days’ notice.

The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, AbbVie, is a customer. The Company identified the following material promises at the outset of the arrangement: (1) a non-exclusive royalty-free research and development license; (2) research and development services for pre-clinical activities under the research plan through to the first research and development phase (or an estimated 17 months); (3) three option rights for AbbVie to continue the collaboration as related to three phases of research and development; (4) participation on the JRC; and (5) the transfer of ownership of IBDCo upon exercise of the option to buy IBDCo. The Company determined that the license and research and development activities were not distinct from one another. Participation on the JRC to oversee the research and development activities was determined to be quantitatively and qualitatively immaterial and therefore is excluded from performance obligations. As such, the Company determined that the license and research and development services should be combined into a single performance obligation.

The Company evaluated the milestone payments, which represent customer options as described above, and the option to purchase IBDCo, to determine whether they provide AbbVie with any material rights. The Company concluded that the options were not issued at a significant and incremental discount, and therefore do not provide material rights. As such, they were excluded as performance obligations at the outset of the arrangement. If AbbVie elects to exercise the options, the additional consideration will be added to the transaction price and allocated to the resulting performance obligations.

Based on these assessments, the Company identified one performance obligation at the outset of the AbbVie Agreement, which consists of: (1) the non-exclusive license and (2) the research and development activities through the first research and development phase.

At the outset of the arrangement, the transaction price includes only the $2.0 million up-front consideration received which was allocated to the single performance obligation. The option exercise fees ($16.5 million for the milestones and the IBDCo purchase option exercise fee) that may be received are excluded from the transaction price until each customer option is exercised. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price.

In May 2017, the Company completed the research and development services for the first phase of the research plan and was paid $2.0 million to commence the second phase of the research plan. At this time, the $2.0 million was added to the transaction price and allocated to a new performance obligation consisting of the underlying license and research and development services to be performed over the second phase of the research plan. Upon the Company’s completion of these activities, AbbVie has the ability to continue the collaboration and make additional payments based on the achievement of specific events outlined in the AbbVie agreement.

Revenue associated with these performance obligations is recognized as the research and development services are provided using an input method, according to the FTEs incurred. The research and development activities are expected to be performed over a period of 54 months. The transfer of control occurs over time and, in management’s judgment, is the best measure of progress towards satisfying the performance obligation. The amounts received that have not yet been recognized as revenue are recorded in deferred revenue on the Company’s condensed consolidated balance sheet.

Through March 31, 2018, the Company had recognized revenue of $3.6 million as collaboration revenue in the Company’s consolidated statements of operations and comprehensive loss under the AbbVie Agreement. Deferred revenue related to the AbbVie Agreement amounted to $0.4 million as of March 31, 2018, all of which is included in current liabilities.

Net Loss per Share/Unit

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Net Loss per Share/Unit

(13)	Net Loss per Share/Unit

Basic net loss per share/unit is computed using the weighted-average number of shares of common stock/units outstanding during the period. Diluted net loss per share/unit is computed using the sum of the weighted-average number of shares of common stock/units outstanding during the period and if dilutive, the weighted-average number of potential shares of common stock/units, including unvested restricted common stock/units and outstanding stock options.

The Company computed basic and diluted net loss per share/unit using the two-class method, which gives effect to the impact of outstanding participating securities. As the three months ended March 31, 2018 and 2017 resulted in net losses attributable to common stockholders/unit holders, there is no income allocation required under the two-class method or dilution attributed to weighted-average shares outstanding in the calculation of diluted net loss per share/unit because the preferred stockholders/unit holders do not participate in losses of the Company. Accordingly, for periods in which the Company reports a net loss attributable to common stockholders/unit holders, diluted net loss per share/unit attributable to common stockholders/unit holders is the same as basic net loss per share/unit attributable to common stockholders/unit holders, since dilutive common stock/units are not assumed to have been issued if their effect is anti-dilutive.

The Company’s potentially dilutive shares/units, which include outstanding stock options and unvested restricted common stock/units, are considered to be common share/unit equivalents and are only included in the calculation of diluted net loss per share/unit when their effect is dilutive.

The following potential common shares/units, presented based on amounts outstanding at each period end, were excluded from the calculation of the diluted net loss per share/unit attributable to common stockholders/unit holders for the period indicated because including them would have had an anti-dilutive effect.

	As of March 31,
	2018		2017
Unvested restricted common unit awards		—		196,425
Unvested restricted common stock awards		320,166		—
Outstanding options to purchase common stock		1,741,527		—

Commitments and Contingencies

3 Months Ended

Mar. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

(14)	Commitments and Contingencies

In the ordinary course of business, the Company may be subject to legal proceedings, claims and litigation as the Company operates in an industry susceptible to patent legal claims. The Company accounts for estimated losses with respect to legal proceedings and claims when such losses are probable and estimable. Legal costs associated with these matters are expensed when incurred. The Company is not currently a party to any material legal proceedings.

The Company’s commitments described in the Company’s consolidated financial statements as of and for the year ended December 31, 2017 and the notes thereto included in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018, have had no material changes during the three months ended March 31, 2018, except as described below.

In July 2017, the Company entered into an agreement to lease approximately 41,346 square feet of laboratory and office space in Cambridge, Massachusetts. Annual rent is $3.1 million. The ten-year lease commenced in January 2018 and contains provisions for a free-rent period, annual rent increases and a landlord-paid allowance for tenant improvements of $6.6 million. In conjunction with the tenant improvements, the Company has made an additional commitment of $2.7 million, of which it has paid $1.7 million. In conjunction with the lease, the Company established a letter of credit of $1.0 million secured by cash balances included in restricted cash on the Company’s consolidated balance sheets.

In July 2017, the Company entered into an agreement to terminate its existing lease of laboratory and office space in Cambridge, Massachusetts at a date that is 30 days after the commencement of its new lease. No penalties are associated with the termination of the lease. As a result of the agreement to terminate its lease, the Company revised its estimate of the remaining amortization period of the deferred rent and its estimate of the remaining useful life our leasehold improvements associated with the 200 Sidney Street facility from 63 months to seven months. The Company vacated the Sidney Street facility in February 2018.

Subsequent Events

3 Months Ended

Mar. 31, 2018

Subsequent Events [Abstract]

Subsequent Events

(15)	Subsequent Events

On April 10, 2018, the Company completed a registered direct offering of 3,280,000 shares at a price of $9.15 per share. After expenses, the Company generated approximately $28.9 million in net proceeds from the offering.

On May 10, 2018, the Company announced the resignation of its President and Chief Executive Officer, Jose-Carlos Gutiérrez-Ramos. The Company expects to incur severance related costs, as well as additional non-cash equity-based compensation expense associated with the modification of certain equity awards.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements and the related disclosures as of March 31, 2018 and for the three months ended March 31, 2018 and 2017 are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) and the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. These interim consolidated financial statements should be read in conjunction with the Company’s 2017 and 2016 audited consolidated financial statements and notes included in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018. The December 31, 2017 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures including notes required by GAAP for complete financial statements. In the opinion of management, the unaudited interim consolidated financial statements reflect all adjustments, consisting of normal and recurring adjustments, necessary for the fair presentation of the Company’s financial position and results of operations for the three months ended March 31, 2018 and 2017. The results of operations for the interim periods are not necessarily indicative of the results to be expected for the year ending December 31, 2018 or any other interim period or future year or period.
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of Synlogic and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements Revenue Recognition In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. This ASU supersedes the revenue recognition requirements in ASC 605, Revenue Recognition, or ASC 605, and creates ASC 606, Revenue from Contracts with Customers. In 2015 and 2016, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients. Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers, or ASC 606, using the modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC 605. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five-step analysis: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step analysis to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The Company has entered into collaboration agreements for research and development services, under which the Company licenses certain rights to its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non-refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. Any variable consideration is constrained, and therefore the cumulative revenue associated with the consideration is not recognized, until it is deemed not be at significant risk of reversal. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under each of its agreements for which the collaboration partner is also a customer, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: a) the number of performance obligations based on the determination under step (ii) above; b) the transaction price under step (iii) above; and c) the contract term and pattern of satisfaction of the performance obligations under step (v) above. The Company uses significant judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to the goods and services the Company expects to provide. The Company uses estimates to determine the timing of satisfaction of performance obligations, which may include the use of full time employees (“FTE”) as a measure of satisfaction of performance obligations. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current deferred revenue. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Licenses of Intellectual Property In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promises, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Research and Development Services If an arrangement is determined to contain a promise or obligation for the company to perform research and development services, the Company must determine whether these services are distinct other promises in the arrangement. In assessing whether the services are distinct from the other promises, the Company considers the capabilities of the customer to perform these same services. In addition, the Company considers whether the customer can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For research and development services that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Customer Options If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, that is, the option to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on an alternative approach when the goods or services are both (i) similar to the original goods and services in the contract and (ii) provided in accordance with the terms of the original contract. Under this alternative, the Company allocates the total amount of consideration expected to be received from the customer to the total goods or services expected to be provided to the customer. Amounts allocated to a material right are not recognized as revenue until the option is exercised and the performance obligation is satisfied. Milestone Payments At the inception of each arrangement that includes milestone payments, the Company evaluates whether a significant reversal of cumulative revenue provided in conjunction with achieving the milestones is probable and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant reversal of cumulative revenue would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. For other milestones, the Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant reversal of cumulative revenue would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Royalties For arrangements that include sales-based royalties, including milestone payments based on a level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. Contract costs The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer. For additional discussion of accounting for collaboration revenues, see Note 12, AbbVie Collaboration Agreement. Impact of Adoption As a result of adopting ASC 606 on January 1, 2018, the Company has recorded a cumulative-effect decrease to opening accumulated deficit of $0.3 million as of January 1, 2018 and a corresponding decrease to deferred revenue. Total revenue recorded in the three months ended March 31, 2018 under ASC 606 was $0.4 million, as compared to $0.1 million that would have been recorded under ASC 605. Deferred revenue as of March 31, 2018 was $0.4 million under ASC 606, as compared to a balance of $1.0 million which would have resulted under ASC 605. The most significant changes relate to the Company’s revenue recognition pattern for the AbbVie Collaboration Agreement and the accounting for milestone payments. Under ASC 605, the Company was recognizing the revenue allocated to each unit of accounting on a straight‑line basis over the period the Company is expected to complete its obligations. Under ASC 606, the Company is recognizing the revenue allocated to each performance obligation measuring progress using an input method over the period the Company is expected to complete each performance obligation. Under ASC 605, the Company recognized revenue related to milestone payments as the milestone was achieved, using the milestone method. Under ASC 606, the Company determined that the milestones at the beginning of certain research and development phases represent a 90-day contract with daily customer renewal options for the Company’s continued research and development services. As a result, revenue from these milestones is recognized over a performance obligation consisting of the next phase of research and development services. For further discussion of the adoption of this standard, see Note 12, AbbVie Collaboration Agreement. Stock Compensation In May 2017, the FASB issued ASU 2017-09, Compensation – Stock Compensation (Topic 718): Scope Modification Accounting. The new standard is intended to reduce the diversity in practice and cost and complexity when applying the guidance in Topic 718 to a change to the terms or conditions of a share-based payment award. The new standard will be effective for annual reporting periods and interim periods within those annual periods, beginning after December 15, 2017. The amendments in this update will be applied prospectively to an award modified on or after the adoption date. The Company adopted this standard as of January 1, 2018 and it did not have a material impact on the Company’s financial position or results of operations.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements The recently issued accounting pronouncements described in the Company’s consolidated financial statements as of and for the year ended December 31, 2017, and the notes thereto included in the Company’s Annual Report on Form 10-K filed with the SEC on March 20, 2018, have had no material changes during the three months ended March 31, 2018, except as described below. In February 2016, the FASB issued ASU 2016-02 – Leases (Topic 842), which replaces the existing accounting guidance for leases. This standard requires entities that lease assets to recognize the assets and liabilities for the rights and obligations created by those leases on the balance sheet. The standard is effective for fiscal years and the interim periods within those fiscal years beginning after December 15, 2018. The guidance is required to be applied by the modified retrospective transition approach and early adoption is permitted. While the Company is currently assessing the impact that adoption of this guidance will have on its financial statements and footnote disclosures, it anticipates it will result in an increase in assets and liabilities. In February 2018, the FASB issued ASU 2018-02 – Income Statement – Reporting Comprehensive Income (Topic 220), which provides amended guidance on income tax accounting. The amended guidance permits the reclassification of the income tax effect on amounts recorded within other comprehensive income impacted by the Tax Cuts and Jobs Act into retained earnings. The amended guidance is effective for periods ending after December 15, 2018 and applies only to those amounts remaining in Other Comprehensive Income at the date of enactment of the Act. The amended guidance may be adopted on either a retrospective basis or at the beginning of the period of adoption. The Company is assessing the potential impact of the amended standard. In March 2018, the FASB issued Accounting Standards Update No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 (“ASU 2018-05”). The standard amends Accounting Standards Codification 740, Income Taxes (“ASC 740”), to provide guidance on accounting for the tax effects of the Tax Act pursuant to Staff Accounting Bulletin No. 118. The Company is currently evaluating the new guidance included in ASU 2018-05, but does not expect it to have a material impact on its consolidated financial statements.

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Company's Classified Assets Measured at Fair Value on Recurring Basis

the Company has classified assets measured at fair value on a recurring basis as follows (in thousands):

	Fair Value Measurements at Reporting Date Using
	March, 31		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	15,816	$	15,816	$	—	$	—
Corporate debt securities (included in cash and cash equivalents)		1,246		—		1,246		—
Corporate debt securities (included in short- term investments)		98,618		—		98,618		—
Total	$	115,680	$	15,816	$	99,864	$	—

	Fair Value Measurements at Reporting Date Using
	December 31,		Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Money market funds (included in cash and cash equivalents)	$	21,301	$	21,301	$	—	$	—
Corporate debt securities (included in cash and cash equivalents)		11,405		—		11,405		—
Corporate debt securities (included in short- term investments)		28,585		—		28,585		—
Total	$	61,291	$	21,301	$	39,990	$	—

Available-for-Sale Investments (Tables)

3 Months Ended

Mar. 31, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Available-for-Sale Securities Held

The following tables summarize the available-for-sale securities held at March 31, 2018 and December 31, 2017 (in thousands):

March 31, 2018

Amortized cost

Gross unrealized

gains

Gross unrealized

losses

Fair Value

Corporate debt securities

98,729

(118

)

98,618

Total

98,729

(118

)

98,618

December 31, 2017

Amortized cost

Gross unrealized

gains

Gross unrealized

losses

Fair Value

Corporate debt securities

28,593

)

28,585

Total

28,593

)

28,585

Property and Equipment, Net (Tables)

3 Months Ended

Mar. 31, 2018

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consists of the following (in thousands):

	March 31,			December 31,
	2018			2017
Laboratory equipment	$	3,936		$	2,999
Computer and office equipment		739			354
Furniture and fixtures		409			220
Leasehold improvements		8,504			2,308
Construction in progress		1,389			7,017
		14,977			12,898
Less accumulated depreciation		(1,273	)		(3,115	)
	$	13,704		$	9,783

Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2018

Payables And Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consists of the following (in thousands):

	March 31,		December 31,
	2018		2017
Payroll related	$	823	$	1,721
Professional fees		671		805
Research and development		1,028		2,027
Other		1,715		270
	$	4,237	$	4,823

Equity-based Compensation and Equity Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2018

Schedule of Restricted Common Stock Activity

The following table shows restricted stock activity during the three months ended March 31, 2018:

	Restricted stock awards
				Grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2017		375,479		$	13.55
Granted		—			—
Vested		(55,313	)		13.53
Forfeited		—			—
Unvested at March 31, 2018		320,166		$	13.56

Schedule of Equity-based Compensation Expenses

	Three months ended March 31,
	2018		2017
Research and development	$	377	$	59
General and administrative		542		71
	$	919	$	130

Schedule of Equity-based Compensation Expenses by Award Type

The following table summarizes equity‑based compensation expense by type of award for the three months ended March 31, 2018 and 2017 (in thousands):

	Three months ended March 31,
	2018		2017
Stock options	$	773	$	—
Restricted stock awards		146		—
Incentive units		—	$	96
Restricted common units		—		34
	$	919	$	130

2015 and 2017 Plan

Schedule of Stock Option Activity

The following table summarizes stock option activity during the three months ended March 31, 2018, as adjusted for the Exchange Ratio under the 2015 Plan and the 2017 Plan.

	Stock options outstanding
						Weighted
				Weighted		average		Aggregate
				average		remaining		Intrinsic
	Number of			exercise		contractual		value^(a)
	options			price		term		(in thousands)
Outstanding at December 31, 2017		1,267,221		$	13.62		9.6	$	—
Granted		479,096			10.01		9.9		—
Exercised		—			—				—
Forfeited		(4,790	)		13.53		9.2
Outstanding at March 31, 2018		1,741,527			12.63		9.5	$	—

Vested or expected to vest at March 31, 2018		1,741,527			12.63		9.5		—

Exercisable at March 31, 2018		198,944			13.62		9.1	$	—

(a)

Net Loss per Share/Unit (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Schedule of Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit

	As of March 31,
	2018		2017
Unvested restricted common unit awards		—		196,425
Unvested restricted common stock awards		320,166		—
Outstanding options to purchase common stock		1,741,527		—

Nature of Business - Additional Information (Detail) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended
	Jan. 31, 2018 USD ($) $ / shares	Jan. 26, 2018 $ / shares shares	Jan. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)
Nature Of Business [Line Items]
Number of operating segment \| Segment				1
Cash, cash equivalents, and marketable securities				$ 125,800
Restricted cash				1,097	$ 1,097
Accumulated deficit				(82,495)	$ (71,654)
Net proceeds from sale of common stock				$ 53,752
Common Stock
Nature Of Business [Line Items]
Stock sold and issued to investors \| shares		5,130,000
Stock sold and issued to investors, per share \| $ / shares		$ 9.75
Net proceeds from sale of common stock	$ 53,800
Common Stock \| Underwritten Public Offering, Including Underwriters’ Allotment
Nature Of Business [Line Items]
Stock sold and issued to investors \| shares			5,899,500
Stock sold and issued to investors, per share \| $ / shares	$ 9.75		$ 9.75
Net proceeds from sale of common stock			$ 53,800

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Revenue, practical expedient, incremental cost of obtaining contract [true/false]	true
Revenue, practical expedient, remaining performance obligation, description	The Company recognizes as an asset the incremental costs of obtaining a contract with a customer if the costs are expected to be recovered. As a practical expedient, the Company recognizes the incremental costs of obtaining a contract as an expense when incurred if the amortization period of the asset that we otherwise would have recognized is one year or less. To date, the Company has not incurred any incremental costs of obtaining a contract with a customer
Cumulative-effect decrease to opening accumulated deficit	$ (82,495)			$ (71,654)
Revenue	354	$ 111
ASC 606
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue	400
Revenue	400
ASC 606 \| Impact as Result of Adoption of ASC 606
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect decrease to opening accumulated deficit			$ 300
Deferred revenue	(300)
ASC 606 \| Balances without Adoption of ASC 606
Summary Of Significant Accounting Policies [Line Items]
Deferred revenue	1,000
Revenue	$ 100

Merger with Mirna Therapeutics - Additional Information (Detail)	Aug. 28, 2017 Director	Aug. 25, 2017	Mar. 31, 2018 shares	Dec. 31, 2017 shares	Aug. 29, 2017 shares
Business Acquisition [Line Items]
Business combination date	Aug. 28, 2017
Reserve stock split of common stock prior to its merger		0.14
Exchange ratio of common stock	0.5532
Common stock outstanding \| shares			22,172,117	16,272,617	16,282,496
Number of members on Board of Directors	7
Synlogic
Business Acquisition [Line Items]
Number of members on Board of Directors	5
Mirna therapeutics \| Common Stock \| Synlogic
Business Acquisition [Line Items]
Share ownership following merger, percent	83.00%

Fair Value of Financial Instruments - Schedule of Company's Classified Assets Measured at Fair Value on Recurring Basis (Detail) - Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	$ 115,680	$ 61,291
Corporate debt securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	1,246	11,405
Short-term investments	98,618	28,585
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	15,816	21,301
Significant Other Observable Inputs (Level 2)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	99,864	39,990
Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	1,246	11,405
Short-term investments	98,618	28,585
Significant Unobservable Inputs (Level 3)
Fair Value Asset Measured On Recurring Basis [Line Items]
Total assets	0	0
Significant Unobservable Inputs (Level 3) \| Corporate debt securities
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	0	0
Short-term investments	0	0
Money market funds
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	15,816	21,301
Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	15,816	21,301
Money market funds \| Significant Unobservable Inputs (Level 3)
Fair Value Asset Measured On Recurring Basis [Line Items]
Cash and Cash Equivalents	$ 0	$ 0

Available-for-Sale Securities - Summary of Available-for-Sale Securities Held (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 98,729	$ 28,593
Gross unrealized gains	7	1
Gross unrealized losses	(118)	(9)
Fair Value	98,618	28,585
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	98,729	28,593
Gross unrealized gains	7	1
Gross unrealized losses	(118)	(9)
Fair Value	$ 98,618	$ 28,585

Available-for-Sale Securities - Additional Information (Detail) $ in Millions	Mar. 31, 2018 USD ($) Investment Security	Dec. 31, 2017 USD ($) Investment
Investments Debt And Equity Securities [Abstract]
Number of investments in unrealized loss position, less than twelve months	30	7
Number of investments in unrealized loss position, more than twelve months	0	0
Aggregate fair value of securities in unrealized loss position \| $	$ 86.3	$ 19.3
Number of securities with other than temporary impairment \| Security	0

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 14,977	$ 12,898
Less accumulated depreciation	(1,273)	(3,115)
Property and equipment, net	13,704	9,783
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	3,936	2,999
Computer and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	739	354
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	409	220
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	8,504	2,308
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,389	$ 7,017

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Leasehold Improvements
Property Plant And Equipment [Line Items]
Lessor-paid tenant improvements	$ 6.6	$ 1.3
Construction in Progress
Property Plant And Equipment [Line Items]
Lessor-paid tenant improvements		5.0
Laboratory Equipment
Property Plant And Equipment [Line Items]
Minimum percentage of fair value on present value of expected payments	90.00%
Assets under capital lease	$ 1.3	1.4
Accumulated depreciation of capital lease	$ 0.2	$ 0.2

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Payroll related	$ 823	$ 1,721
Professional fees	671	805
Research and development	1,028	2,027
Other	1,715	270
Total accrued expenses	$ 4,237	$ 4,823

Accrued Expenses - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Line Items]
Other accrued expenses	$ 1,715,000	$ 270,000
Construction in Progress
Payables And Accruals [Line Items]
Other accrued expenses	$ 1,300,000	$ 31,000

Common Stock - Additional Information (Detail)			3 Months Ended	12 Months Ended	124 Months Ended
Common Stock - Additional Information (Detail)	Jan. 31, 2018 USD ($) shares	Jan. 26, 2018 $ / shares shares	Mar. 31, 2018 USD ($) Vote shares	Dec. 31, 2017 shares	Mar. 31, 2018 USD ($) shares
Class Of Stock [Line Items]
Number of votes entitled to each share of common stock \| Vote			1
Cash dividends \| $					$ 0
Proceeds from sale of common stock, net of issuance costs \| $			$ 53,752,000
Common Stock
Class Of Stock [Line Items]
Stock sold and issued to investors		5,130,000
Stock sold and issued to investors, per share \| $ / shares		$ 9.75
Proceeds from sale of common stock, net of issuance costs \| $	$ 53,800,000
Repurchase or forfeiture rights share			360,527		360,527
Repurchase option exercisable period			4 years
Repurchase options share exercised			41,819	41,819
Stock purchase rights forfeited			16,111	16,111
Common Stock \| Over-Allotment Option
Class Of Stock [Line Items]
Stock sold and issued to investors	769,500

Preferred Stock - Additional Information (Detail)	3 Months Ended
Preferred Stock - Additional Information (Detail)	Mar. 31, 2018 shares	Dec. 31, 2017 shares	Aug. 28, 2017
Preferred Stock [Abstract]
Preferred stock, shares issued	0	0
Exchange ratio of common stock			0.5532
Preferred stock, conversion basis	one-for-one
Preferred stock, conversion percentage	100.00%

Equity-based Compensation and Equity Incentive Plans - Additional Information (Detail)		3 Months Ended
	Jan. 01, 2018 shares	Mar. 31, 2018 USD ($) Plan shares	Mar. 31, 2017 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity plan currently active \| Plan		2
Granted \| shares			0
Equity-based compensation expense		$ 919,000	$ 130,000
Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense		146,000
Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		0
Units issued \| shares			0
Equity-based compensation expense			$ 96,000
Restricted Common Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued \| shares			0
Unrecognized compensation expense		0
Equity-based compensation expense			$ 34,000
Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		$ 11,400,000
Unrecognized compensation cost, period of recognition		3 years 1 month 6 days
Employee \| Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		$ 500,000
Unrecognized compensation cost, period of recognition		1 year 10 months 24 days
Nonemployee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		$ 100,000
Unrecognized compensation cost, period of recognition		8 months 12 days
Nonemployee \| Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		$ 33,000
Unrecognized compensation cost, period of recognition		4 months 24 days
Plan 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase in number of shares available for issuance (in shares) \| shares	813,630
2015 and 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants \| shares		425,133
Granted \| shares		479,096

Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 and 2017 Plan (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Number of options		0
2015 and 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding Beginning balance, Number of options	1,267,221
Granted, Number of options	479,096
Forfeited, Number of options	(4,790)
Outstanding Ending balance, Number of options	1,741,527		1,267,221
Number of options, Vested or expected to vest at March 31, 2018	1,741,527
Number of options, Exercisable at March 31, 2018	198,944
Beginning balance, Weighted-average price	$ 13.62
Granted, Weighted-average price	10.01
Forfeited, Weighted-average price	13.53
Ending balance, Weighted-average price	12.63		$ 13.62
Weighted average price Vested or expected to vest at March 31, 2018	12.63
Weighted average price, Exercisable at March 31, 2018	$ 13.62
Weighted average remaining contractual term	9 years 6 months		9 years 7 months 6 days
Granted, Weighted average remaining contractual term	9 years 10 months 24 days
Forfeited, Weighted average remaining contractual term	9 years 2 months 12 days
Weighted average remaining contractual term, Vested or expected to vest at March 31, 2018	9 years 6 months
Weighted average remaining contractual term, Exercisable at March 31, 2018	9 years 1 month 6 days
Beginning balance, Aggregate Intrinsic value	$ 0
Ending balance, Aggregate Intrinsic value	$ 0		$ 0

Equity-based Compensation and Equity Incentive Plans - Schedule of Stock Option Activity Under 2015 and 2017 Plan (Parenthetical) (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
2015 and 2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, aggregate intrinsic value	$ 0	$ 0

Equity-based Compensation and Equity Incentive Plans - Schedule of Restricted Stock Activity (Detail) - Restricted Common Stock	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Beginning balance, Number of unvested shares/units \| shares	375,479
Vested, Number of shares/units \| shares	(55,313)
Ending balance, Number of unvested shares/units \| shares	320,166
Beginning balance, Unvested Grant date fair value \| $ / shares	$ 13.55
Vested, Grant date fair value \| $ / shares	13.53
Ending balance, Unvested Grant date fair value \| $ / shares	$ 13.56

Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 919	$ 130
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	377	59
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 542	$ 71

Equity-based Compensation and Equity Incentive Plans - Schedule of Equity-based Compensation Expenses by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 919	$ 130
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	773
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense	$ 146
Incentive Units
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense		96
Restricted Common Units
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Equity-based compensation expense		$ 34

AbbVie Collaboration Agreement - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		33 Months Ended
	May 31, 2017	Dec. 31, 2015	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognition of revenue			$ 354	$ 111
Deferred revenue included in current liabilities			434		$ 434	$ 444
AbbVie Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognition of revenue					3,600
Deferred revenue included in current liabilities			400		$ 400
AbbVie Agreement \| IBDCo
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront non refundable payment		$ 2,000
Potential milestone payment			$ 16,500
Collaboration in research and development			54 months
Milestone payment	$ 2,000

- Definition

The carrying amount of consideration received or receivable as of the balance sheet date on potential earnings that were not recognized as revenue in conformity with GAAP, and which are expected to be recognized as such within one year or the normal operating cycle, if longer, including sales, license fees, and royalties, but excluding interest income.

+ References + Details

Net Loss per Share/Unit - Schedule of Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit (Detail) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Unvested Restricted Common Unit Awards
Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit		196,425
Unvested Restricted Common Stock Awards
Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit	320,166
Stock Options
Potentially Common Shares/Units Excluded from Calculation of Net Loss Per share/Unit	1,741,527

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended		3 Months Ended
	Jan. 31, 2018 USD ($)	Jul. 31, 2017 USD ($) ft²	Mar. 31, 2018 USD ($)
Asset Purchase Agreement And Related License Agreement [Abstract]
Laboratory and office space to be leased \| ft²		41,346
Operating Lease Annual Rent		$ 3,100,000
Term of lease		10 years
Payment for tenant improvement	$ 6,600,000
Additional tenant improvement commitment			$ 2,700,000
Tenant improvements paid by lessee			$ 1,700,000
Letter of credit		$ 1,000,000
Agreement termination term		30 days
Penalty on contract termination		$ 0
Remaining amortization period of deferred rent and remaining useful life leasehold improvements		63 months
Revised amortization period of deferred rent and revised useful life leasehold improvements		7 months

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 10, 2018	Mar. 31, 2018
Subsequent Event [Line Items]
Net proceeds from offering		$ 53,752
Subsequent Event \| Direct Offering
Subsequent Event [Line Items]
Stock sold and issued	3,280,000
Stock sold and issued, per share	$ 9.15
Net proceeds from offering	$ 28,900