Seres Therapeutics (MCRB) 10-K 6 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 28, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	1,609,809
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		41,048,360
Entity Public Float			$ 177,045,629

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 85,820,000	$ 36,088,000
Investments	0	113,895,000
Prepaid expenses and other current assets	6,845,000	5,095,000
Total current assets	92,665,000	155,078,000
Property and equipment, net	26,294,000	32,931,000
Restricted investments	1,400,000	1,400,000
Restricted cash	113,000	113,000
Total assets	120,472,000	189,522,000
Current liabilities:
Accounts payable	6,415,000	7,033,000
Accrued expenses and other current liabilities	15,207,000	12,513,000
Deferred revenue - related party	20,419,000	12,079,000
Total current liabilities	42,041,000	31,625,000
Lease incentive obligation, net of current portion	7,250,000	8,989,000
Deferred rent	2,216,000	2,233,000
Deferred revenue, net of current portion - related party	116,840,000	84,847,000
Other long-term liabilities	170,000	1,129,000
Total liabilities	168,517,000	128,823,000
Commitments and contingencies (Note 11)
Stockholders’ equity (deficit):
Preferred stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2018 and 2017; no shares issued and outstanding at December 31, 2018 and 2017
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2018 and 2017; 40,936,735 and 40,571,015 shares issued and outstanding at December 31, 2018 and 2017	41,000	40,000
Additional paid-in capital	341,284,000	324,376,000
Accumulated other comprehensive income (loss)		(146,000)
Accumulated deficit	(389,370,000)	(263,571,000)
Total stockholders’ equity (deficit)	(48,045,000)	60,699,000
Total liabilities and stockholders’ equity (deficit)	$ 120,472,000	$ 189,522,000

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	40,936,735	40,571,015
Common stock, shares outstanding	40,936,735	40,571,015

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Collaboration revenue - related party	$ 26,917	$ 32,100	$ 21,766
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue	$ 1,350
Total revenue	28,267	$ 32,100	$ 21,766
Operating expenses:
Research and development expenses	95,955	89,455	81,989
General and administrative expenses	32,596	34,040	32,616
Total operating expenses	128,551	123,495	114,605
Loss from operations	(100,284)	(91,395)	(92,839)
Other income (expense):
Interest income (expense), net	1,172	1,590	1,260
Other income	170	425
Total other income (expense), net	1,342	2,015	1,260
Net loss	$ (98,942)	$ (89,380)	$ (91,579)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.43)	$ (2.21)	$ (2.30)
Weighted average common shares outstanding, basic and diluted	40,743,492	40,449,410	39,846,928
Other comprehensive income (loss):
Unrealized gain (loss) on investments, net of tax of $0	$ 146	$ 3	$ (179)
Total other comprehensive income (loss)	146	3	(179)
Comprehensive loss	$ (98,796)	$ (89,377)	$ (91,758)

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Unrealized (loss) gain on investment, tax	$ 0	$ 0	$ 0

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2015	$ 205,394	$ 39	$ 287,937	$ 30	$ (82,612)
Beginning balance, shares at Dec. 31, 2015		39,082,017
Issuance of common stock upon exercise of stock options, value	2,138	$ 1	2,137
Issuance of common stock upon exercise of stock options, shares		1,273,736
Stock-based compensation expense	16,857		16,857
Unrealized gain (loss) on investments	(179)			(179)
Net loss	(91,579)				(91,579)
Ending balance at Dec. 31, 2016	132,631	$ 40	306,931	(149)	(174,191)
Ending balance, shares at Dec. 31, 2016		40,355,753
Issuance of common stock upon exercise of stock options, value	116		116
Issuance of common stock upon exercise of stock options, shares		174,386
Issuance of common stock upon vesting of RSUs, net of tax withholdings	(33)		(33)
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		40,876
Stock-based compensation expense	17,362		17,362
Unrealized gain (loss) on investments	3			3
Net loss	(89,380)				(89,380)
Ending balance at Dec. 31, 2017	$ 60,699	$ 40	324,376	(146)	(263,571)
Ending balance, shares at Dec. 31, 2017	40,571,015	40,571,015
Issuance of common stock upon exercise of stock options, value	$ 147	$ 1	146
Issuance of common stock upon exercise of stock options, shares	212,240	212,240
Issuance of common stock upon vesting of RSUs, net of tax withholdings	$ 61		61
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		138,048
Repurchase of common stock for employee tax withholdings	(197)		(197)
Repurchase of common stock for employee tax withholdings, shares		(17,900)
Issuance of common stock under ESPP plan, value	257		257
Issuance of common stock under ESPP plan, shares		33,332
Stock-based compensation expense	16,641		16,641
Unrealized gain (loss) on investments	146			146
Adoption of new revenue standard (ASC 606)	(26,857)				(26,857)
Net loss	(98,942)				(98,942)
Ending balance at Dec. 31, 2018	$ (48,045)	$ 41	$ 341,284	$ 0	$ (389,370)
Ending balance, shares at Dec. 31, 2018	40,936,735	40,936,735

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (98,942)	$ (89,380)	$ (91,579)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	16,641	17,362	16,857
Depreciation and amortization expense	7,862	7,259	4,205
Non-cash interest expense		3
Accretion of discount on investments	(214)	(216)	(386)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,056)	31	(2,598)
Deferred revenue	13,476	(11,888)	108,814
Accounts payable	(353)	(856)	3,581
Accrued expenses and other liabilities	732	2,162	5,027
Net cash provided by (used in) operating activities	(62,854)	(75,523)	43,921
Cash flows from investing activities:
Purchases of property and equipment	(1,937)	(4,676)	(21,492)
Purchases of investments	(21,832)	(97,934)	(290,594)
Sales and maturities of investments	136,087	158,312	246,494
Net cash provided by (used in) investing activities	112,318	55,702	(65,592)
Cash flows from financing activities:
Proceeds from exercise of stock options	147	116	2,138
Proceeds from issuance of common stock and restricted common stock	61
Payments for repurchase of common stock for employee tax withholdings	(197)	(33)
Issuance of common stock under ESPP plan	257
Net cash provided by financing activities	268	83	2,138
Net increase (decrease) in cash, cash equivalents and restricted cash	49,732	(19,738)	(19,533)
Cash, cash equivalents and restricted cash at beginning of year	36,201	55,939	75,472
Cash, cash equivalents and restricted cash at end of year	85,933	36,201	55,939
Supplemental disclosure of cash flow information:
Cash paid for interest		1	109
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 157	$ 877	$ 1,650

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Nature of the Business and Basis of Presentation	1. Seres Therapeutics, Inc. (the “Company”) was incorporated under the laws of the State of Delaware in October 2010 under the name Newco LS21, Inc. In October 2011, the Company changed its name to Seres Health, Inc., and in May 2015, the Company changed its name to Seres Therapeutics, Inc. The Company is a microbiome therapeutics platform company developing a novel class of biological drugs, which are designed to restore health by repairing the function of a dysbiotic microbiome. SER-287 is being developed by the Company to treat ulcerative colitis (“UC”). The Company’s lead product candidate, SER-109, is designed to prevent further recurrences of Clostridium difficile infection (“CDI”), a debilitating infection of the colon, and, if approved by the U.S. Food and Drug Administration (“FDA”), could be a first-in-field oral microbiome drug. The Company is also developing product candidates to treat diseases where the microbiome is implicated, including SER-301, a rationally designed, fermented inflammatory bowel disease (“IBD”), candidate, and SER-401, a microbiome therapeutic candidate for use with checkpoint inhibitors (“CPI’s”) in patients with metastatic melanoma. The Company continues to evaluate microbiome pharmacokinetic and pharmacodynamic data from their SER-262 Phase 1b study and other completed clinical trials, in addition to insights gained from research efforts with their other rationally designed Ecobiotic microbiome therapeutic candidates, in order to determine next steps in the development of both SER-262 to treat an initial recurrence of CDI and SER-155 to modulate the microbiome and dysbiosis in patients following allogeneic hematopoietic stem cell transplants. The Company is also using its microbiome therapeutics platform to conduct research on various indications, including: infectious diseases, metabolic diseases, and inflammatory and immune diseases and cancer. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in development. 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 government 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 significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Under Accounting Standards Update (“ASU”) 2014-15, Presentation of Financial Statements—Going Concern As of December 31, 2018, the Company had an accumulated deficit of $389.4 million and cash and cash equivalents of $85.8 million. For the year ended December 31, 2018, the Company incurred a loss of $98.9 million and used $62.9 million of cash in operations. The Company’s current financial resources and currently forecasted operating plan would allow the Company to operate into the fourth quarter of 2019 which is within one year from the issuance of these consolidated financial statements. The Company has developed plans to mitigate this risk, which primarily consist of raising additional capital through some combination of equity or debt financings, and/or potentially new collaborations and reducing cash expenditures. If the Company is not able to secure adequate additional funding, the Company plans to make reductions in spending. In that event, the Company may have to delay, scale back, or eliminate some or all of the Company’s planned clinical trials and research stage programs. The actions necessary to reduce spending under this plan at a level that mitigates the factors described above is not considered probable, as defined in the accounting standards; as such, under the requirements of ASC 205-40, the full extent to which management may extend the Company’s funds through these actions may not be considered in management’s assessment of the Company’s ability to continue as a going concern for the next 12 months as defined by ASC-205-40. As a result, in accordance with the requirements of ASC 205-40, management has concluded that it is required to disclose that substantial doubt exists about the Company’s ability to continue as a going concern for one year from the date these financial statements are issued. The Company is eligible to receive contingent milestone payments under its license and collaboration agreement with Nestec Ltd. (“NHS”), an affiliate of Nestlé Health Science US Holdings, Inc. (“Nestlé Health Science”), a significant stockholder of the Company, if certain development milestones are achieved. However, these milestones are uncertain and there is no assurance that the Company will receive any of them. Until such time, if ever, as the Company can generate substantial product revenue, the Company will finance its cash needs through a combination of public or private equity offerings, debt financings, governmental funding, collaborations, strategic partnerships or marketing, distribution or licensing arrangements with third parties. The Company may not be able to obtain funding on acceptable terms, or at all. If the Company is unable to raise additional funds as and when needed, it would have a negative impact on the Company’s financial condition, which may require the Company to delay, reduce or eliminate certain research and development activities and reduce or eliminate discretionary operating expenses, which could constrain the Company’s ability to pursue its business strategies. The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of the Company and its wholly owned subsidiaries after elimination of all intercompany accounts and transactions.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts and corporate bonds purchased with original maturities of less than 90 days from the date of purchase, are stated at fair value. Restricted Cash The Company held cash of $113 as of December 31, 2018 and December 31, 2017 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted cash on its balance sheet. Investments The Company classifies its available-for-sale marketable debt securities as current assets on the consolidated balance sheet if they mature within one year from the balance sheet date. The Company classifies all of its marketable debt securities as available-for-sale securities. Accordingly, these marketable debt securities are recorded at fair value and unrealized gains and losses are reported as a separate component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations and comprehensive loss. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the consolidated statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented. Restricted Investments The Company held investments of $1,400 as of December 31, 2018 and December 31, 2017 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted investments on its balance sheet. Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company has all cash, cash equivalents and investments balances at accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above. The Company’s investments in certificates of deposit are carried at amortized cost, which approximates fair value. Certain cash equivalents or investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment and furniture and office equipment are depreciated over three years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations. Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials. Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Accounting for Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options, restricted stock units and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For stock options or restricted stock units issued with performance-based vesting conditions, the stock compensation expense related to these awards is recognized based on the grant date fair value when achievement of the performance condition is deemed probable. The Company measures stock-based awards granted to consultants and non-employees based on the fair value of the award on the date on which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. The Company recognizes adjustments to stock compensation expense for forfeitures as they occur. The fair value of each stock option grant is estimated on the date of grant using the Black- Scholes option-pricing model. The Company lacks sufficient company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. We use the simplified method prescribed by Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share-Based Payment Revenue Recognition The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 while the reported results for 2017 and 2016 were prepared under ASC 605, Revenue Recognition (“ASC 605”). The Company recognizes revenue in accordance with the guidance under ASC 606, Revenue from Contracts with Customers (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 model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services transferred 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 the contract to determine whether each promised good or service is a performance obligation. The promised goods or services in the Company’s arrangements typically consist of a license to the Company’s intellectual property and/or research and development services. The Company may provide options to additional items in such arrangements, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. Performance obligations are promises in a contract to transfer a distinct good or service to the customer that (i) the customer can benefit from on its own or together with other readily available resources, and (ii) is separately identifiable from other promises in the contract. Goods or services that are not individually distinct performance obligations are combined with other promised goods or services until such combined group of promises meet the requirements of a performance obligation. The Company determines transaction price based on the amount of consideration the Company expects to receive for transferring the promised goods or services in the contract. Consideration may be fixed, variable, or a combination of both. At contract inception for arrangements that include variable consideration, the Company estimates the probability and extent of consideration it expects to receive under the contract utilizing either the most likely amount method or expected amount method, whichever best estimates the amount expected to be received. The Company then considers any constraints on the variable consideration and includes in the transaction price variable consideration to the extent it is deemed probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. The Company then allocates the transaction price to each performance obligation based on the relative standalone selling price and recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) control is transferred to the customer and the performance obligation is satisfied. For performance obligations which consist of licenses and 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. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The Company records amounts as accounts receivable when the right to consideration is deemed unconditional. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded for deferred revenue. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. Incremental costs of obtaining a contract are expensed as and when incurred if the expected period over which the Company would have amortized the asset is one year or less, or the amount is immaterial. Collaboration Revenue Arrangements with collaborators may include licenses to intellectual property, research and development services, manufacturing services for clinical and commercial supply, and participation on joint steering committees. The Company evaluates the promised goods or services to determine which promises, or group of promises, represent performance obligations. In contemplation of whether a promised good or service meets the criteria required of a performance obligation, the Company considers the stage of development of the underlying intellectual property, the capabilities and expertise of the customer relative to the underlying intellectual property, and whether the promised goods or services are integral to or dependent on other promises in the contract. When accounting for an arrangement that contains multiple performance obligations, the Company must develop judgmental assumptions, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success to determine the stand-alone selling price for each performance obligation identified in the contract. When the Company concludes that a contract should be accounted for as a combined performance obligation and recognized over-time, the Company must then determine the period over which revenue should be recognized and the method by which to measure revenue. The Company generally recognizes revenue using a cost-based input method. Licenses of intellectual property If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled 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 associated with the bundled performance obligation. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of progress and related revenue recognition. Milestone Payments At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service, otherwise it will be allocated to all performance obligations of the arrangement based on the initial allocation. The Company evaluates each milestone to determine when and how much of the milestone to include in the transaction price. The Company first estimates the amount of the milestone payment that the Company could receive using either the expected value or the most likely amount approach. The Company primarily uses the most likely amount approach as that approach is generally most predictive for milestone payments with a binary outcome. Then, the Company considers whether any portion of that estimated amount is subject to the variable consideration constraint (that is, whether it is probable that a significant reversal of cumulative revenue would not occur upon resolution of the uncertainty). The Company updates the estimate of variable consideration included in the transaction price at each reporting date which includes updating the assessment of the likely amount of consideration and the application of the constraint to reflect current facts and circumstances. Royalties For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize 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 revenue related to sales-based royalties or milestone payments based on the level of sales. Manufacturing supply services For arrangements that include a promise of supply of clinical or commercial product, the Company determines if the supply is a promise in the contract or a future obligation at the customer’s option. If determined to be a promise at inception of the contract, the Company evaluates the promise to determine whether it is a separate performance obligation or a component of a bundled performance obligation. If determined to be an option, the Company determines if the option provides a material right to the customer and if so, accounts for the option as a separate performance obligation. If determined to be an option but not a material right, the Company accounts for the option as a separate contract when the customer elects to exercise the option. Grant The Company generates revenue from government contracts that reimburse the Company for certain allowable costs for funded projects. For contracts with government agencies, when the Company has concluded that it is the principal in conducting the research and development expenses and where the funding arrangement is considered central to the Company’s ongoing operations, the Company classifies the recognized funding received as revenue. The Company has concluded to recognize funding received as revenue, rather than as a reduction of research and development expenses, because the Company is the principal in conducting the research and development activities and these contracts are central to its ongoing operations. Revenue is recognized as the qualifying expenses related to the contracts are incurred. Revenue recognized upon incurring qualifying expenses in advance of receipt of funding is recorded in the Company’s consolidated balance sheet as prepaid expenses and other current assets. The related costs incurred by the Company are included in research and development expense in the Company’s consolidated statements of operations and comprehensive loss. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing Ecobiotic microbiome therapeutics to treat dysbiosis in the colonic microbiome. Revenue to date has been generated solely through the Company's collaboration with NHS, all of which has been earned in the United States. All tangible assets are held in the United States. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2018, 2017 and 2016, other comprehensive income (loss) consisted of changes in unrealized gains (losses) from available-for-sale investments. Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and unvested restricted stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders, as its convertible preferred stock was, and common stock is a participating security. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock as the Company was in a net loss position for each of the periods presented and preferred stockholders did not participate in losses. The Company’s convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Recently Issued and Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU 2016-18, Restricted Cash December 31 December December December 2018 2017 2016 2015 Cash and cash equivalents $ 85,820 $ 36,088 $ 54,539 $ 73,933 Restricted cash 113 113 1,400 1,539 Total cash, cash equivalents and restricted cash as shown in the statement of cash flows $ 85,933 $ 36,201 $ 55,939 $ 75,472 In February 2016 the FASB issued ASU No. 2016-02, Leases (Topic 842) The FASB has subsequently issued the following amendments to ASU 2016-02, which have the same effective date and transition date of January 1, 2019, and which the Company collectively refers to as the new leasing standards: • ASU No. 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842, Leases • ASU No. 2018-10, Codification Improvements to Topic 842, Leases • ASU No. 2018-11, Leases (Topic 842): Targeted Improvements • ASU No. 2018-20, Narrow-Scope Improvements for Lessors The Company adopted the new leasing standards on January 1, 2019, using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019. The Company elected to apply the package of practical expedients which allows entities not to reassess whether contracts are or contain leases, lease classification, and whether initial direct costs qualify for capitalization. Additionally, the Company elected not to separate lease and non-lease components. The Company’s primary operating leases represent the lease of its corporate headquarters at 200 Sidney Street and a lease for additional research space at 215 First Street, both in Cambridge, Massachusetts. Upon adoption of the new leasing standards, the Company expects to recognize a lease liability of approximately $25,000 and a related right-of-use asset of approximately $14,000 on its consolidated balance sheet with the difference being due to the elimination of previously reported lease incentive obligations and deferred rent. The impact of adoption of the new leasing standards will have an immaterial impact to the Company’s consolidated statements of operations and comprehensive loss and cash flows. In June 2018, the FASB issued ASU 2018-07, Compensation – Stock Compensation (Topic 718) In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s assets and liabilities as of December 31, 2018 and 2017 that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2018 Using: Level 1 Level 2 Level 3 Not Leveling (1) Total Assets: Cash Equivalents $ — $ — $ — $ 39,982 $ 39,982 $ — $ — $ — $ 39,982 $ 39,982 (1) Fair Value Measurements as of December 31, 2017 Using: Level 1 Level 2 Level 3 Not Subject to Leveling (1) Total Assets: Cash Equivalents $ — $ — $ — $ 25,964 $ 25,964 Investments: Commercial Paper — 6,198 — — 6,198 Certificates of Deposit — 8,916 — — 8,916 Corporate Bonds — 58,865 — — 58,865 Government Securities — 22,954 — — 22,954 Treasury Bonds — 16,961 — — 16,961 $ — $ 113,894 $ — $ 25,964 $ 139,858 (1) Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. As of December 31, 2018 and 2017, the Company’s cash equivalents consisted of money market funds with original maturities of less than 90 days from the date of purchase. Excluded from the tables above are restricted cash and restricted investments as the cost approximates current fair value. The Company has a money market fund and certificate of deposit as security deposits on its two facility leases. As of both December 31, 2018 and 2017 the restricted cash and restricted investment balances were $113 and $1,400, respectively. The fair value of the Company’s investments, which consisted of corporate bonds, commercial paper, certificates of deposit, government securities, and treasury bonds as of December 31, 2017 were determined using Level 2 inputs.

Investments

Investments	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments As of December 31, 2018 the Company had no investments. As of December 31, 2017, the fair value of available-for-sale investments by type of security was as follows: December 31, 2017 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 6,198 $ — $ — $ 6,198 Certificates of Deposit 8,916 — — 8,916 Corporate Bonds 58,937 — (72 ) 58,865 Government Securities 22,997 — (43 ) 22,954 Treasury Bonds 16,992 — (31 ) 16,961 $ 114,040 $ — $ (146 ) $ 113,894 Investments with original maturities of less than 90 days are included in cash and cash equivalents on the consolidated balance sheets and are not included in the table above. Investments with maturities of less than 12 months are considered current and those investments with maturities greater than 12 months are considered non-current. Excluded from the table above are restricted investments of $1,400 as the cost approximates current fair value.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following: December 31, 2018 December 31, 2017 Laboratory equipment $ 14,695 $ 13,181 Computer equipment 2,864 2,832 Furniture and office equipment 1,033 1,033 Leasehold improvements 27,977 27,963 Construction in progress 26 361 46,595 45,370 Less: Accumulated depreciation and amortization (20,301 ) (12,439 ) $ 26,294 $ 32,931 Depreciation and amortization expense was $7,862, $7,259 and $4,205 for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: December 31, 2018 December 31, 2017 Development and clinical manufacturing costs $ 7,046 $ 3,910 Payroll and payroll-related costs 5,020 4,962 Facility and other 3,141 3,641 $ 15,207 $ 12,513

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Convertible Preferred Stock	7 . Convertible Preferred Stock On July 1, 2015, in connection with the closing of the initial public offering of the Company’s common stock (“IPO”), the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 10,000,000 shares of preferred stock, $0.001 par value per share.

Stockholders' Equity Common Sto

Stockholders' Equity Common Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity Common Stock	8. Stockholders’ Equity Common Stock On July 1, 2015, in connection with the closing of the IPO, the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 200,000,000 shares of common stock, $0.001 par value per share. 2012 Stock Incentive Plan The Company’s 2012 Stock Incentive Plan, as amended, (the “2012 Plan”) provided for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2012 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally granted stock-based awards with service conditions only (“service-based” awards). Stock options granted under the 2012 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2018, there were no shares available for future grant under the 2012 Plan. 2015 Incentive Award Plan On June 16, 2015, the Company’s stockholders approved the 2015 Incentive Award Plan (the “2015 Plan”), which became effective on June 25, 2015. The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors. Stock options granted under the 2015 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2018, there were 1,353,580 shares available for future grant under the 2015 Plan. 2015 Employee Stock Purchase Plan On June 16, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective on June 25, 2015. A total of 365,000 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP automatically increase on the first day of each calendar year, beginning in 2016 and ending in 2025, by an amount equal to the least of (i) 400,000 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (iii) an amount determined by the Company’s board of directors. Offering periods under the ESPP will commence when determined by the plan administrator. As of December 31, 2018, there were 33,332 shares issued under the ESPP and 1,522,488 shares were reserved and available for issuance The ESPP provides that eligible employees may contribute up to 15% of their eligible earnings toward the semi-annual purchase of the Company's common stock. The ESPP is qualified under Section 423 of the Internal Revenue Code. The employee's purchase price is derived from a formula based on the closing price of the common stock on the first day of the offering period versus the closing price on the date of purchase (or, if not a trading day, on the immediately preceding trading day). The offering period under the ESPP has a duration of six months, and the purchase price with respect to each offering period beginning on or after such date is, until otherwise amended, equal to 85% of the lesser of (i) the fair market value of the Company's common stock at the commencement of the applicable six-month offering period or (ii) the fair market value of the Company's common stock on the purchase date. The Company estimates the fair value of common stock under the ESPP using a Black-Scholes valuation model. The fair value was estimated on the date of grant using the Black-Scholes option valuation model and the straight-line attribution approach with the following weighted-average assumptions: risk-free interest rate (2.30%); expected term (0.5 years); expected volatility (60.2%); and an expected dividend yield (0%). The Company recorded $145 of stock-based compensation expense under the ESPP for the twelve months ended December 31, 2018. There was no stock-based compensation expense under the ESPP for the twelve months ended December 31, 2017, and 2016 respectively. Stock Option Valuation The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.39 % 2.20 % 1.54 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 76.0 % 80.9 % 84.2 % Expected dividend yield 0 % 0 % 0 % Stock Options The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2018: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2017 6,125,692 $ 13.00 7.70 $ 15,808 Granted 2,074,776 9.67 Exercised (212,240 ) 0.69 Forfeited (426,509 ) 15.99 Outstanding as of December 31, 2018 7,561,719 $ 12.26 7.23 $ 4,958 Options vested and expected to vest as of December 31, 2018 7,561,719 $ 12.26 7.23 $ 4,958 Options exercisable as of December 31, 2018 4,386,848 $ 12.56 6.21 $ 4,958 The weighted average grant-date fair value of stock options granted during the years ended December 31, 2018, 2017 and 2016 was $6.53, $7.07, and $18.45 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2018, 2017, and 2016 was $1,801, $1,912, and $34,871. The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The Company granted performance-based stock options to certain employees. These stock options were exercisable only upon achievement of specified performance targets. During the year ended December 31, 2016, the Company determined that the achievement of specified performance targets occurred. Upon achievement of the performance targets, 60,000 options became immediately vested. The Company recorded stock-based compensation expense of $235 during the year ended December 31, 2016 related to these awards. The grant date fair value of these awards was $3.92 per share. As of December 31, 2018 and 2017, there were outstanding unvested service-based stock options held by non-employees for the purchase of 193,282 and 45,000 shares, respectively, of common stock. Restricted Stock The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted stock units as of December 31, 2017 356,778 $ 10.01 Granted 30,900 $ 7.50 Forfeited (22,730 ) $ 9.87 Vested (138,048 ) $ 10.09 Unvested restricted stock units as of December 31, 2018 226,900 $ 9.64 The aggregate intrinsic value of restricted stock units that vested during the years ended December 31, 2018, 2017 and 2016 was $1,206, $445, and $0, respectively. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2018 2017 2016 Research and development expenses $ 8,388 $ 8,115 $ 8,310 General and administrative expenses 8,253 9,247 8,547 $ 16,641 $ 17,362 $ 16,857 As of December 31, 2018, the Company had an aggregate of $21,554 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.19 years.

Collaboration Revenue

Collaboration Revenue	12 Months Ended
	Dec. 31, 2018
Revenue From Contract With Customer [Abstract]
Collaboration Revenue	9. Collaboration Revenue The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 while the reported results for 2017 and 2016 were prepared under ASC 605. Adoption of ASC Topic 606, Revenue from Contracts with Customers The adoption of ASC 606 changed the pattern and timing of revenue recognition under the Collaboration and License Agreement (“License Agreement”) with NHS Under ASC 605, the upfront fee of $120,000 received by the Company in the first quarter of 2016 was deferred and recognized on a straight-line basis over the estimated performance period of ten years. In addition, the Company recognized revenue associated with substantive development milestones not considered probable at the inception of the License Agreement with NHS in their entirety in the period in which the milestone was achieved, in accordance with ASC 605-28, Revenue Recognition-Milestone Method . Under ASC 606, the Company recognizes revenue using the cost-to-cost method which best depicts the transfer of control to the customer. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimate of the Company’s measure of progress and estimate of transaction price will be updated at each reporting date, as a change in estimate, and will require judgement. The amount of consideration allocated to satisfied performance obligations, based on the Company’s measure of progress will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time. In addition, substantive development milestones, which were previously recognized in the period when the milestone was achieved, will be recognized over the remaining performance period under ASC 606. The cumulative effect of applying ASC 606 as of the adoption date of January 1, 2018, was recorded as an adjustment to accumulated deficit in the statement of stockholders’ equity (deficit). As a result of applying the modified retrospective method to adopt ASC 606, the following adjustments were made to the consolidated balance sheet on January 1, 2018: As Reported at Adjustments Due to Balance at December 31, 2017 ASC 606 January 1, 2018 Deferred revenue - related party 12,079 5,339 17,418 Deferred revenue, net of current portion - related party 84,847 21,518 106,365 Accumulated deficit (263,571 ) (26,857 ) (290,428 ) The following table compares the reported consolidated balance sheet as of December 31, 2018 to the pro-forma amounts had ASC 605 been in effect : As of December 31, 2018 As Reported under ASC 606 Pro forma as if accounted for under ASC 605 Deferred revenue - related party 20,419 12,118 Deferred revenue, net of current portion - related party 116,840 73,004 Accumulated deficit (389,370 ) (337,233 ) Total reported liabilities were $52,137 greater as reported under ASC 606 than would have been reported under ASC 605 as of December 31, 2018. This change consists of a $43,836 increase in deferred revenue, net of current portion, related party, and a $8,301 increase in deferred revenue - related parties. The following table compares the reported consolidated statements of operations and comprehensive loss for the year ended December 31, 2018 to the pro forma amounts had ASC 605 been in effect: Year Ended December 31, 2018 As Reported under ASC 606 Pro forma as if accounted for under ASC 605 Collaboration revenue - related party $ 26,917 $ 52,197 Loss from operations $ (100,284 ) (75,004 ) Net loss $ (98,942 ) (73,662 ) Net loss per share attributable to common stockholders, basic and diluted $ (2.43 ) $ (1.81 ) Collaboration revenue – related party decreased by approximately $25,280 as reported under ASC 606 when compared to what would have been reported under ASC 605 for the year ended December 31, 2018. The following table compares the reported consolidated statements of cash flows for the year ended December 31, 2018 to the pro forma amounts had ASC 605 been in effect: Year Ended December 31, 2018 As Reported under ASC 606 Pro forma as if accounted for under ASC 605 Net loss $ (98,942 ) $ (73,662 ) Deferred revenue $ 13,476 (11,804 ) The adoption of ASC 606 resulted in a $25,280 increase in net loss, due to a corresponding decrease in revenue and an increase of $25,280 in the change in deferred revenue for the year ended December 31, 2018. The adoption of ASC 606 did not change net cash used in operating activities. NHS Collaboration Agreement Summary of Agreement In January 2016, the Company entered into the License Agreement for the development and commercialization of certain product candidates in development for the treatment and management of CDI and IBD, including UC and Crohn’s disease. The License Agreement supports the development of the Company’s portfolio of products for CDI and IBD in markets outside of the United States and Canada (the “Licensed Territory”). The Company has retained full commercial rights to its entire portfolio of product candidates with respect to the United States and Canada. Under the License Agreement, the Company granted to NHS an exclusive, royalty-bearing license to develop and commercialize, in the Licensed Territory, certain products based on its microbiome technology that are being developed for the treatment of CDI and IBD, including SER-109, SER-262, SER-287 and SER-301 (collectively, the “NHS Collaboration Products”). The License Agreement sets forth the Company’s and NHS’ respective obligations for development, commercialization, regulatory and manufacturing and supply activities for the NHS Collaboration Products with respect to the licensed fields and the Licensed Territory. Under the License Agreement, NHS agreed to pay the Company an upfront cash payment of $120,000, which the Company received in February 2016. The Company is eligible to receive up to $285,000 in development milestone payments, $375,000 in regulatory payments and up to an aggregate of $1,125,000 for the achievement of certain commercial milestones related to the sales of NHS Collaboration Products. NHS also agreed to pay the Company tiered royalties, at percentages ranging from the high single digits to high teens, of net sales of NHS Collaboration Products in the Licensed Territory. Under the License Agreement, the Company is entitled to receive a $20,000 milestone payment from NHS following initiation of a SER-287 Phase 2 study and a $20,000 milestone payment from NHS following the initiation of a SER-287 Phase 3 study. In November 2018, the Company entered into a letter agreement with NHS which modified the License Agreement to address the current clinical plans for SER-287. Through the letter agreement, the Company and NHS agreed that following initiation of the SER-287 Phase 2b study, the Company will receive $40,000 in milestone payments from NHS which represents the milestone due to the Company for the initiation of the SER-287 Phase 2 and Phase 3 studies. The SER-287 Phase 2b study was initiated and the $40,000 of milestone payments were received in December 2018. The letter agreement also provides scenarios under which NHS’ reimbursement to the Company for certain Phase 3 development costs would be reduced or delayed depending on the outcomes of the SER-287 Phase 2b study. Accounting Analysis The Company assessed the License Agreement in accordance with ASC 606 and concluded that NHS is a customer. The Company identified the following promises under the contract: (i) a license to develop and commercialize the NHS Collaboration Products in the Licensed Territory, (ii) obligation to perform research and development services, (iii) participation on a joint steering committee, and (iv) manufacturing services to provide clinical supply to complete future clinical trials. In addition, the Company identified a contingent obligation to perform manufacturing services to provide commercial supply if commercialization occurs, which is contingent upon regulatory approval. This contingent obligation is not a performance obligation at inception and has been excluded from the initial allocation as it represents a separate buying decision at market rates, rather than a material right in the contract. The Company assessed the promised goods and services to determine if they are distinct. Based on this assessment, the Company determined that NHS cannot benefit from the promised goods and services separately from the others as they are highly interrelated and therefore not distinct. Accordingly, the promised goods and services represent one combined performance obligation and the entire transaction price will be allocated to that single combined performance obligation. At contract inception, the Company determined that the $120,000 non-refundable upfront amount constituted the entirety of the consideration to be included in the transaction price as the development, regulatory, and commercial milestones were fully constrained. During the year ended December 31, 2016, the Company received $10,000 from NHS in connection with the initiation of the Phase 1b study for SER-262 in CDI. During the year ended December 31, 2017, the Company received $20,000 from NHS in connection with the initiation of the Phase 3 study for SER-109. Therefore, as of January 1, 2018, the aggregate amount of the transaction price allocated to the remaining performance obligation of the License Agreement was approximately $150,000. During the year ended December 31, 2018, the Company received $40,000 from NHS in connection with the initiation of the Phase 2b study for SER-287. In the third quarter of 2018, the Company increased the transaction price by $20,000 associated with the Phase 2 milestone for SER-287. The Company estimated the $20,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled. In the fourth quarter of 2018, the Company entered into a letter agreement with NHS which modified the License Agreement to address the current clinical plans for SER-287. As a result of this modification, the Company and NHS agreed that the $20,000 milestone payment due to the Company from NHS following the initiation of a Phase 3 study for SER-287 would now be due to the Company upon initiation of the SER-287 Phase 2b study. The Letter Agreement constituted a contract modification under ASC 606. The Company accounted for the contract modification through a cumulative catch-up adjustment because the contract modification did not add any additional goods or services and the remaining goods and services are not distinct. The SER-287 Phase 2b study was initiated in December 2018 and the Company included the $20,000 in the transaction price as of December 31, 2018. The transaction price as of December 31, 2018 was approximately $190,000. During the twelve months ended December 31, 2018, using the cost-to-cost method, w hich best depicts the transfer of control to the customer, For the twelve months ended December 31, 2017, the Company recognized $32,100 of Collaboration revenue – related party under ASC 605 which comprised (i) a $20,000 milestone for the initiation of the Phase 3 study for SER-109, recognized in accordance with the milestone method of revenue recognition (ASC 605-28) and (ii) $12,100 of revenue from the upfront license fee. For the twelve months ended December 31, 2016, the Company recognized $21,766 of Collaboration revenue – related party under ASC 605 which comprised (i) a $10,000 milestone for the initiation of the Phase 3 study for SER-109, recorded in accordance with the milestone method of revenue recognition (ASC 605-28) and (ii) $11,766 of revenue from the upfront license fee. As of December 31, 2018 and 2017, there was $137,259, and $96,926 of deferred revenue related to the unsatisfied portion of the performance obligation under the License Agreement. As of December 31, 2018, the deferred revenue is classified as current or non-current in the consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next twelve months which is determined by the cost-to-cost method which measures the extent of progress towards completion based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the performance obligation. All costs associated with the License Agreement are recorded in research and development expense in the consolidated statements of operations and comprehensive loss. Income Taxes The adoption of ASC 606 resulted in an increase to deferred revenue, which in turn generated an additional deferred tax asset that increased the Company’s net deferred tax asset position. As the Company fully reserves its net deferred tax assets, the impact was offset by the valuation allowance. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract liabilities during the twelve months ended December 31, 2018: Balance as of January 1, 2018 Additions Deductions Balance as of December 31, 2018 Year ended December 31, 2018 Contract liabilities: Deferred revenue 123,783 40,393 (26,917 ) 137,259 During the twelve months ended December 31, 2018 we recognized the following revenues as a result of changes in the contract liability balances in the respective periods: Year Ended December 31, 2018 Revenue recognized in the period from: Amounts included in the contract liability at the beginning of the period 15,774 When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Revenue is recognized from the contract liability over time using the cost-to-cost method.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	10 . Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2018 2017 2016 Numerator: Net loss attributable to common stockholders $ (98,942 ) $ (89,380 ) $ (91,579 ) Denominator: Weighted average common shares outstanding, basic and diluted 40,743,492 40,449,410 39,846,928 Net loss per share attributable to common stockholders, basic and diluted $ (2.43 ) $ (2.21 ) $ (2.30 ) The Company’s potential dilutive securities, which include stock options, unvested restricted common stock and shares issuable under the ESPP, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2018 2017 2016 Stock options to purchase common stock 7,561,719 6,125,692 5,069,133 Unvested restricted stock units 226,900 356,778 115,500 Shares issuable under ESPP 49,495 — — 7,838,114 6,482,470 5,184,633

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11 . Commitments and Contingencies Leases The Company previously leased office and laboratory space with a lease term expiring in January 2018 and no extension periods. In May 2016, upon mutual agreement with the landlord, the Company accelerated the termination of the operating lease to June 30, 2016. Upon termination of the lease, the Company recorded a benefit to rent expense of $136 to write off amounts previously recorded as deferred rent. The outstanding security deposit of $119, which was secured by a cash collateralized letter of credit, was released in September 2016. On April 1, 2015, the Company entered into a lease for additional office and laboratory space with a term expiring in April 2020. On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016. BMR has contributed a total of $12,509 toward the cost of tenant improvements. BMR’s contributions toward the cost of tenant improvements is recorded as a lease incentive obligation on the Company’s consolidated balance sheet. The lease incentive obligation is amortized to the Company’s consolidated statement of operations as reductions to rent expense over the lease term. As of December 31, 2018, the Company recorded a lease incentive obligation of $8,544. During the years ended December 31, 2018, 2017 and 2016, the Company recognized $4,377, $4,458, and $3,826, respectively, of rental expense related to office and laboratory space. Future minimum lease payments for these operating leases as of December 31, 2018 are as follows: Year Ending December 31, 2019 $ 6,342 2020 6,120 2021 6,221 2022 6,372 2023 5,158 2024 and thereafter — Total $ 30,213 Indemnification Agreements In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2018 or 2017.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	12 . Income Taxes During the years ended December 31, 2018, 2017 and 2016, the Company recorded no income tax benefits for the net operating losses incurred in each year or interim period, due to its uncertainty of realizing a benefit from those items. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2018 2017 2016 Federal statutory income tax rate (21.0 )% (34.0 )% (34.0 )% Research and development tax credits (7.2 ) (7.7 ) (8.8 ) State taxes, net of federal benefit (7.2 ) (4.8 ) (5.8 ) Stock-based compensation 2.0 1.2 (6.3 ) Revaluation of preferred stock warrant liability — 33.1 — Other 0.8 (0.3 ) 0.1 Change in deferred tax asset valuation allowance 32.6 12.5 54.8 Effective income tax rate — % — % — % As permitted by SEC Staff Accounting Bulletin 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“TCJA”), the Company recorded provisional estimates during the year ended December 31, 2017 and have subsequently finalized its accounting analysis based on the guidance, interpretations, and data available as of December 31, 2018. The Company finalized its accounting for the income tax effects of the TCJA during 2018, with no adjustment to the provisional amounts previously recorded. Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following: December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 50,723 $ 32,997 Research and development tax credit carryforwards 32,522 25,405 Capitalized organization costs 275 306 Stock-based compensation expense 9,178 7,714 Charitable Contributions 11 8 Deferred Revenue 37,499 26,480 Accrued expenses 4,279 4,613 Capitalized research and development expenses 66 73 Total deferred tax assets $ 134,553 $ 97,596 Deferred tax liabilities: Depreciation and amortization (2,544 ) (3,470 ) Total deferred tax liabilities (2,544 ) (3,470 ) Valuation allowance $ (132,009 ) $ (94,126 ) Net deferred tax assets $ — $ — As of December 31, 2018, the Company had net operating loss carryforwards (“NOLs”) for federal and state income tax purposes of $185,583 and $185,927, respectively. Federal NOLs of $119,781, generated before 2018, will begin expiring in varying amounts in 2035 unless utilized. The remaining federal NOLs of $65,801, generated after 2017, will be carried forward indefinitely and could be used to offset up to 80% of taxable income of each future tax year. Massachusetts does not follow federal time periods for NOLs and as such the Company’s Massachusetts NOLs of $185,927 will expire in at various times starting in 2035. As of December 31, 2018, the Company also had available research and development tax credit carryforwards for federal and state income tax purposes of $28,638 and $4,916, respectively, which begin to expire in 2031 and 2028, respectively. The federal research and development tax credits include an orphan drug credit carryforward of $17,552. Utilization of the NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50% over a three-year period. Since our formation, the Company has raised capital through the issuance of capital stock on several occasions. These financings, combined with the purchasing shareholders' subsequent disposition of those shares, may have resulted in a change of control as defined by Section 382 or could result in a change of control in the future upon subsequent disposition. The Company conducted an analysis under Section 382 to determine if historical changes in ownership through August 31, 2015 would limit or otherwise restrict its ability to utilize these NOLs and research and development credit carryforwards. As a result of this analysis, the Company does not believe there are any significant limitations on its ability to utilize these carryforwards. However, future changes in ownership after August 31, 2015 could affect the limitation in future years. Any limitation may result in expiration of a portion of the NOLs or research and development credit carryforwards before utilization. The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2018 and 2017. Management reevaluates the positive and negative evidence at each reporting period. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018, 2017 and 2016 related primarily to the increases in NOLs, research and development tax credit carryforwards, stock-based compensation and decrease of the deferred rate due to tax reform were as follows: Year Ended December 31, 2018 2017 2016 Valuation allowance at beginning of year $ (94,126 ) $ (82,994 ) $ (32,777 ) Decreases recorded as benefit to income tax provision — 29,546 — Increases recorded to income tax provision (37,883 ) (40,678 ) (50,217 ) Valuation allowance as of end of year $ (132,009 ) $ (94,126 ) $ (82,994 ) As of December 31, 2018, 2017, and 2016, the Company had no unrecognized tax benefits. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense. The Company does not expect any significant change in its uncertain tax positions in the next 12 months. During the year ended December 31, 2016, the Company was awarded a tax incentive for job creation from the Massachusetts Life Sciences Center. The program was established in 2008 in order to incentivize life sciences companies to create new sustained jobs in Massachusetts. Jobs must be maintained for at least five years, during which time the grant proceeds can be recovered by the Massachusetts Department of Revenue if the Company does not meet and maintain its job creation commitments.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Selected Quarterly Financial Data (Unaudited)	13 . Selected Quarterly Financial Data (Unaudited) The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party (1) $ 3,766 $ 4,271 $ 8,684 $ 10,196 $ 26,917 Grant revenue 205 341 371 433 1,350 Total operating expenses 32,237 32,748 31,266 32,300 128,551 Loss from operations (28,266 ) (28,136 ) (22,211 ) (21,671 ) (100,284 ) Net loss (27,919 ) (27,787 ) (21,949 ) (21,287 ) (98,942 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.69 ) $ (0.68 ) $ (0.54 ) $ (0.52 ) $ (2.43 ) 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party (1) $ 3,015 $ 3,014 $ 23,015 $ 3,056 $ 32,100 Total operating expenses 28,905 31,430 30,329 32,831 123,495 Loss from operations (25,890 ) (28,416 ) (7,314 ) (29,775 ) (91,395 ) Net loss (25,474 ) (28,018 ) (6,935 ) (28,953 ) (89,380 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.63 ) $ (0.69 ) $ (0.17 ) $ (0.72 ) $ (2.21 ) (1) The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The Company’s reported results for 2018 reflect the application of ASC 606 guidance while its reported results for 2017 and 2016 were prepared under the guidance outlined in ASC 605. Refer to Note 9 for further details.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	14 . Related Party Transactions In October 2010, the Company entered into a services agreement with Flagship Ventures Management, Inc., an affiliate of one of its stockholders (and now known as Flagship Pioneering), Flagship Venture Funds, to provide general and administrative services to the Company, including the employer portions of employee health and dental benefit plans for the Company’s employees and consulting services. The Company made payments under the agreement of $17 during the year ended December 31, 2016. There were no payments made during the year ended December 31, 2018 and 2017. There were no amounts due to Flagship Pioneering related to the services agreement as of December 31, 2018 and 2017. As described in Note 9, in January 2016 the Company entered into a License Agreement and, in November 2018, a letter agreement with NHS for the development and commercialization of certain product candidates in development for the treatment and management of CDI and IBD, including UC and Crohn’s disease. NHS is a related party since NHS is an affiliate of Nestlé Health Science, one of the Company’s significant stockholders. During the year ended December 31, 2018, 2017, and 2016, the Company recognized $26,917, $32,100, and $21,766, respectively, of related party revenue associated with the License Agreement. As of December 31, 2018 and 2017, there was $137,259 and $96,926, respectively, of deferred revenue related to the License Agreement, which is classified as current or non-current in the consolidated balance sheets. The Company has made no payments to NHS during the year ended December 31, 2018. There is no amount due from NHS as of December 31, 2018.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	15 . 401(k) Savings Plan The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Effective January 1, 2016, the Company elected to match 50% of the first 6% of an employee’s deferral. Company contributions are expensed in the year for which they are declared. During the year ended December 31, 2018, 2017, and 2016 the Company recorded expense of $604, $484, and $457, respectively for 401(k) match contributions.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	16. Subsequent Events On January 14, 2019, Roger J. Pomerantz, M.D. notified the Company of his resignation from his position as President and Chief Executive Officer of the Company effective immediately. In connection with his resignation, Dr. Pomerantz entered into a separation agreement and release (the “Separation Agreement”) with the Company pursuant to which Dr. Pomerantz will receive the severance payments and benefits described in his existing employment agreement with the Company upon his involuntary or constructive termination, subject to and in accordance with the previously disclosed terms of his employment agreement. Dr. Pomerantz will continue to serve as a director and as Chairman of the Board of Directors (the “Board”) of the Company. The Company expects to record a liability in the first quarter of 2019 to reflect its obligations to Dr. Pomerantz under the Separation Agreement. On January 15, 2019, the Company announced that the Board had appointed Eric D. Shaff, as the Company’s President and Chief Executive Officer and as a member of the Board. On February 7, 2019, the Company announced corporate changes to focus its resources on advancing its clinical-stage therapeutics candidates. As a result, the Company will now concentrate on completing the recently-initiated SER-287 Phase 2b study in mild-to-moderate UC, obtaining results from the ongoing SER-109 Phase 3 study for recurrent CDI and advancing the SER-401 Phase 1b study, in collaboration with the Parker Institute for Cancer Immunotherapy and MD Anderson Cancer Center, to evaluate augmenting checkpoint inhibitor response in patients with metastatic melanoma. In connection with the prioritization of these therapeutics candidates, the Company made changes to its management team and committed to a workforce reduction plan to reduce headcount by approximately 30 employees. Accordingly, the Company expects to incur one-time costs related primarily to employee termination benefits between $1,500 and $2,000.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Cash Equivalents	Cash Equivalents The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which consist of money market accounts and corporate bonds purchased with original maturities of less than 90 days from the date of purchase, are stated at fair value.
Restricted Cash	Restricted Cash The Company held cash of $113 as of December 31, 2018 and December 31, 2017 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted cash on its balance sheet.
Investments	Investments The Company classifies its available-for-sale marketable debt securities as current assets on the consolidated balance sheet if they mature within one year from the balance sheet date. The Company classifies all of its marketable debt securities as available-for-sale securities. Accordingly, these marketable debt securities are recorded at fair value and unrealized gains and losses are reported as a separate component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense) within the consolidated statement of operations and comprehensive loss. If any adjustment to fair value reflects a decline in the value of the investment that the Company considers to be “other than temporary”, the Company reduces the investment to fair value through a charge to the consolidated statement of operations and comprehensive loss. No such adjustments were necessary during the periods presented.
Restricted Investments	Restricted Investments The Company held investments of $1,400 as of December 31, 2018 and December 31, 2017 in a separate restricted bank account as a security deposit for the lease of the Company’s facilities. The Company has classified these deposits as long-term restricted investments on its balance sheet.
Concentration of Credit Risk and of Significant Suppliers	Concentration of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and investments. The Company has all cash, cash equivalents and investments balances at accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company is dependent on third-party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. These programs could be adversely affected by a significant interruption in the supply of such drug substance products.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above. The Company’s investments in certificates of deposit are carried at amortized cost, which approximates fair value. Certain cash equivalents or investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Laboratory equipment is depreciated over five years. Computer equipment and furniture and office equipment are depreciated over three years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long-lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long-lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials.
Research Contract Costs and Accruals	Research Contract Costs and Accruals The Company has entered into various research and development contracts with research institutions and other companies. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Accounting for Stock-Based Compensation	Accounting for Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees and directors based on the fair value on the date of the grant and recognizes compensation expense of those awards over the requisite service period, which is generally the vesting period of the respective award. Generally, the Company issues stock options, restricted stock units and restricted stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For stock options or restricted stock units issued with performance-based vesting conditions, the stock compensation expense related to these awards is recognized based on the grant date fair value when achievement of the performance condition is deemed probable. The Company measures stock-based awards granted to consultants and non-employees based on the fair value of the award on the date on which the related service is complete. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in its consolidated statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. The Company recognizes adjustments to stock compensation expense for forfeitures as they occur. The fair value of each stock option grant is estimated on the date of grant using the Black- Scholes option-pricing model. The Company lacks sufficient company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. We use the simplified method prescribed by Securities and Exchange Commission Staff Accounting Bulletin No. 107, Share-Based Payment
Revenue Recognition	Revenue Recognition The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The reported results for 2018 reflect the application of ASC 606 while the reported results for 2017 and 2016 were prepared under ASC 605, Revenue Recognition (“ASC 605”). The Company recognizes revenue in accordance with the guidance under ASC 606, Revenue from Contracts with Customers (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 model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services transferred 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 the contract to determine whether each promised good or service is a performance obligation. The promised goods or services in the Company’s arrangements typically consist of a license to the Company’s intellectual property and/or research and development services. The Company may provide options to additional items in such arrangements, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. Performance obligations are promises in a contract to transfer a distinct good or service to the customer that (i) the customer can benefit from on its own or together with other readily available resources, and (ii) is separately identifiable from other promises in the contract. Goods or services that are not individually distinct performance obligations are combined with other promised goods or services until such combined group of promises meet the requirements of a performance obligation. The Company determines transaction price based on the amount of consideration the Company expects to receive for transferring the promised goods or services in the contract. Consideration may be fixed, variable, or a combination of both. At contract inception for arrangements that include variable consideration, the Company estimates the probability and extent of consideration it expects to receive under the contract utilizing either the most likely amount method or expected amount method, whichever best estimates the amount expected to be received. The Company then considers any constraints on the variable consideration and includes in the transaction price variable consideration to the extent it is deemed probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. The Company then allocates the transaction price to each performance obligation based on the relative standalone selling price and recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) control is transferred to the customer and the performance obligation is satisfied. For performance obligations which consist of licenses and 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. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The Company records amounts as accounts receivable when the right to consideration is deemed unconditional. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded for deferred revenue. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. Incremental costs of obtaining a contract are expensed as and when incurred if the expected period over which the Company would have amortized the asset is one year or less, or the amount is immaterial.
Collaborative Revenue	Collaboration Revenue Arrangements with collaborators may include licenses to intellectual property, research and development services, manufacturing services for clinical and commercial supply, and participation on joint steering committees. The Company evaluates the promised goods or services to determine which promises, or group of promises, represent performance obligations. In contemplation of whether a promised good or service meets the criteria required of a performance obligation, the Company considers the stage of development of the underlying intellectual property, the capabilities and expertise of the customer relative to the underlying intellectual property, and whether the promised goods or services are integral to or dependent on other promises in the contract. When accounting for an arrangement that contains multiple performance obligations, the Company must develop judgmental assumptions, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success to determine the stand-alone selling price for each performance obligation identified in the contract. When the Company concludes that a contract should be accounted for as a combined performance obligation and recognized over-time, the Company must then determine the period over which revenue should be recognized and the method by which to measure revenue. The Company generally recognizes revenue using a cost-based input method. Licenses of intellectual property If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. For licenses that are bundled 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 associated with the bundled performance obligation. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of progress and related revenue recognition. Milestone Payments At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service, otherwise it will be allocated to all performance obligations of the arrangement based on the initial allocation. The Company evaluates each milestone to determine when and how much of the milestone to include in the transaction price. The Company first estimates the amount of the milestone payment that the Company could receive using either the expected value or the most likely amount approach. The Company primarily uses the most likely amount approach as that approach is generally most predictive for milestone payments with a binary outcome. Then, the Company considers whether any portion of that estimated amount is subject to the variable consideration constraint (that is, whether it is probable that a significant reversal of cumulative revenue would not occur upon resolution of the uncertainty). The Company updates the estimate of variable consideration included in the transaction price at each reporting date which includes updating the assessment of the likely amount of consideration and the application of the constraint to reflect current facts and circumstances. Royalties For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize 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 revenue related to sales-based royalties or milestone payments based on the level of sales. Manufacturing supply services For arrangements that include a promise of supply of clinical or commercial product, the Company determines if the supply is a promise in the contract or a future obligation at the customer’s option. If determined to be a promise at inception of the contract, the Company evaluates the promise to determine whether it is a separate performance obligation or a component of a bundled performance obligation. If determined to be an option, the Company determines if the option provides a material right to the customer and if so, accounts for the option as a separate performance obligation. If determined to be an option but not a material right, the Company accounts for the option as a separate contract when the customer elects to exercise the option.
Grant Revenue	Grant The Company generates revenue from government contracts that reimburse the Company for certain allowable costs for funded projects. For contracts with government agencies, when the Company has concluded that it is the principal in conducting the research and development expenses and where the funding arrangement is considered central to the Company’s ongoing operations, the Company classifies the recognized funding received as revenue. The Company has concluded to recognize funding received as revenue, rather than as a reduction of research and development expenses, because the Company is the principal in conducting the research and development activities and these contracts are central to its ongoing operations. Revenue is recognized as the qualifying expenses related to the contracts are incurred. Revenue recognized upon incurring qualifying expenses in advance of receipt of funding is recorded in the Company’s consolidated balance sheet as prepaid expenses and other current assets. The related costs incurred by the Company are included in research and development expense in the Company’s consolidated statements of operations and comprehensive loss.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. The Company accounts for uncertainty in income taxes recognized in the financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.
Segment Data	Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing Ecobiotic microbiome therapeutics to treat dysbiosis in the colonic microbiome. Revenue to date has been generated solely through the Company's collaboration with NHS, all of which has been earned in the United States. All tangible assets are held in the United States.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the years ended December 31, 2018, 2017 and 2016, other comprehensive income (loss) consisted of changes in unrealized gains (losses) from available-for-sale investments.
Net Loss per Share	Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and unvested restricted stock. The Company applies the two-class method to calculate its basic and diluted net loss per share attributable to common stockholders, as its convertible preferred stock was, and common stock is a participating security. The two-class method is an earnings allocation formula that treats a participating security as having rights to earnings that otherwise would have been available to common stockholders. However, the two-class method does not impact the net loss per share of common stock as the Company was in a net loss position for each of the periods presented and preferred stockholders did not participate in losses. The Company’s convertible preferred stock contractually entitled the holders of such shares to participate in dividends but did not contractually require the holders of such shares to participate in losses of the Company. Similarly, restricted stock awards granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU 2016-18, Restricted Cash December 31 December December December 2018 2017 2016 2015 Cash and cash equivalents $ 85,820 $ 36,088 $ 54,539 $ 73,933 Restricted cash 113 113 1,400 1,539 Total cash, cash equivalents and restricted cash as shown in the statement of cash flows $ 85,933 $ 36,201 $ 55,939 $ 75,472 In February 2016 the FASB issued ASU No. 2016-02, Leases (Topic 842) The FASB has subsequently issued the following amendments to ASU 2016-02, which have the same effective date and transition date of January 1, 2019, and which the Company collectively refers to as the new leasing standards: • ASU No. 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842, Leases • ASU No. 2018-10, Codification Improvements to Topic 842, Leases • ASU No. 2018-11, Leases (Topic 842): Targeted Improvements • ASU No. 2018-20, Narrow-Scope Improvements for Lessors The Company adopted the new leasing standards on January 1, 2019, using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019. The Company elected to apply the package of practical expedients which allows entities not to reassess whether contracts are or contain leases, lease classification, and whether initial direct costs qualify for capitalization. Additionally, the Company elected not to separate lease and non-lease components. The Company’s primary operating leases represent the lease of its corporate headquarters at 200 Sidney Street and a lease for additional research space at 215 First Street, both in Cambridge, Massachusetts. Upon adoption of the new leasing standards, the Company expects to recognize a lease liability of approximately $25,000 and a related right-of-use asset of approximately $14,000 on its consolidated balance sheet with the difference being due to the elimination of previously reported lease incentive obligations and deferred rent. The impact of adoption of the new leasing standards will have an immaterial impact to the Company’s consolidated statements of operations and comprehensive loss and cash flows. In June 2018, the FASB issued ASU 2018-07, Compensation – Stock Compensation (Topic 718) In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	A reconciliation of the cash, cash equivalents, and restricted cash reported within the balance sheet that sum to the total of the same amounts shown in the statement of cash flows is as follows: December 31 December December December 2018 2017 2016 2015 Cash and cash equivalents $ 85,820 $ 36,088 $ 54,539 $ 73,933 Restricted cash 113 113 1,400 1,539 Total cash, cash equivalents and restricted cash as shown in the statement of cash flows $ 85,933 $ 36,201 $ 55,939 $ 75,472

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets and liabilities as of December 31, 2018 and 2017 that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values: Fair Value Measurements as of December 31, 2018 Using: Level 1 Level 2 Level 3 Not Leveling (1) Total Assets: Cash Equivalents $ — $ — $ — $ 39,982 $ 39,982 $ — $ — $ — $ 39,982 $ 39,982 (1) Fair Value Measurements as of December 31, 2017 Using: Level 1 Level 2 Level 3 Not Subject to Leveling (1) Total Assets: Cash Equivalents $ — $ — $ — $ 25,964 $ 25,964 Investments: Commercial Paper — 6,198 — — 6,198 Certificates of Deposit — 8,916 — — 8,916 Corporate Bonds — 58,865 — — 58,865 Government Securities — 22,954 — — 22,954 Treasury Bonds — 16,961 — — 16,961 $ — $ 113,894 $ — $ 25,964 $ 139,858 (1) Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Schedule of Fair Value of Available-for-Sale Investments by Type of Security	As of December 31, 2017, the fair value of available-for-sale investments by type of security was as follows: December 31, 2017 Amortized Cost Gross Unrealized Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 6,198 $ — $ — $ 6,198 Certificates of Deposit 8,916 — — 8,916 Corporate Bonds 58,937 — (72 ) 58,865 Government Securities 22,997 — (43 ) 22,954 Treasury Bonds 16,992 — (31 ) 16,961 $ 114,040 $ — $ (146 ) $ 113,894

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following: December 31, 2018 December 31, 2017 Laboratory equipment $ 14,695 $ 13,181 Computer equipment 2,864 2,832 Furniture and office equipment 1,033 1,033 Leasehold improvements 27,977 27,963 Construction in progress 26 361 46,595 45,370 Less: Accumulated depreciation and amortization (20,301 ) (12,439 ) $ 26,294 $ 32,931

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: December 31, 2018 December 31, 2017 Development and clinical manufacturing costs $ 7,046 $ 3,910 Payroll and payroll-related costs 5,020 4,962 Facility and other 3,141 3,641 $ 15,207 $ 12,513

Stockholders' Equity Common S_2

Stockholders' Equity Common Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Summary of Fair Value of Stock Option	The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.39 % 2.20 % 1.54 % Expected term (in years) 6.0 6.0 6.0 Expected volatility 76.0 % 80.9 % 84.2 % Expected dividend yield 0 % 0 % 0 %
Summary of Stock Option Activity	The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2018: Number of Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) Outstanding as of December 31, 2017 6,125,692 $ 13.00 7.70 $ 15,808 Granted 2,074,776 9.67 Exercised (212,240 ) 0.69 Forfeited (426,509 ) 15.99 Outstanding as of December 31, 2018 7,561,719 $ 12.26 7.23 $ 4,958 Options vested and expected to vest as of December 31, 2018 7,561,719 $ 12.26 7.23 $ 4,958 Options exercisable as of December 31, 2018 4,386,848 $ 12.56 6.21 $ 4,958
Summary of Restricted Stock Activity	The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2018: Number of Shares Weighted Average Grant Date Fair Value Unvested restricted stock units as of December 31, 2017 356,778 $ 10.01 Granted 30,900 $ 7.50 Forfeited (22,730 ) $ 9.87 Vested (138,048 ) $ 10.09 Unvested restricted stock units as of December 31, 2018 226,900 $ 9.64
Summary of Stock Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss: Year Ended December 31, 2018 2017 2016 Research and development expenses $ 8,388 $ 8,115 $ 8,310 General and administrative expenses 8,253 9,247 8,547 $ 16,641 $ 17,362 $ 16,857

Collaboration Revenue (Tables)

Collaboration Revenue (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue From Contract With Customer [Abstract]
Adjustments to Consolidated Balance Sheet	The cumulative effect of applying ASC 606 as of the adoption date of January 1, 2018, was recorded as an adjustment to accumulated deficit in the statement of stockholders’ equity (deficit). As a result of applying the modified retrospective method to adopt ASC 606, the following adjustments were made to the consolidated balance sheet on January 1, 2018: As Reported at Adjustments Due to Balance at December 31, 2017 ASC 606 January 1, 2018 Deferred revenue - related party 12,079 5,339 17,418 Deferred revenue, net of current portion - related party 84,847 21,518 106,365 Accumulated deficit (263,571 ) (26,857 ) (290,428 )
Comparison of Reported Consolidated Balance Sheets, Statements of Operations and Comprehensive Loss and Cash Flows to Pro-forma Amounts	The following table compares the reported consolidated balance sheet as of December 31, 2018 to the pro-forma amounts had ASC 605 been in effect : As of December 31, 2018 As Reported under ASC 606 Pro forma as if accounted for under ASC 605 Deferred revenue - related party 20,419 12,118 Deferred revenue, net of current portion - related party 116,840 73,004 Accumulated deficit (389,370 ) (337,233 ) The following table compares the reported consolidated statements of operations and comprehensive loss for the year ended December 31, 2018 to the pro forma amounts had ASC 605 been in effect: Year Ended December 31, 2018 As Reported under ASC 606 Pro forma as if accounted for under ASC 605 Collaboration revenue - related party $ 26,917 $ 52,197 Loss from operations $ (100,284 ) (75,004 ) Net loss $ (98,942 ) (73,662 ) Net loss per share attributable to common stockholders, basic and diluted $ (2.43 ) $ (1.81 ) The following table compares the reported consolidated statements of cash flows for the year ended December 31, 2018 to the pro forma amounts had ASC 605 been in effect: Year Ended December 31, 2018 As Reported under ASC 606 Pro forma as if accounted for under ASC 605 Net loss $ (98,942 ) $ (73,662 ) Deferred revenue $ 13,476 (11,804 )
Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the twelve months ended December 31, 2018: Balance as of January 1, 2018 Additions Deductions Balance as of December 31, 2018 Year ended December 31, 2018 Contract liabilities: Deferred revenue 123,783 40,393 (26,917 ) 137,259 During the twelve months ended December 31, 2018 we recognized the following revenues as a result of changes in the contract liability balances in the respective periods: Year Ended December 31, 2018 Revenue recognized in the period from: Amounts included in the contract liability at the beginning of the period 15,774

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows: Year Ended December 31, 2018 2017 2016 Numerator: Net loss attributable to common stockholders $ (98,942 ) $ (89,380 ) $ (91,579 ) Denominator: Weighted average common shares outstanding, basic and diluted 40,743,492 40,449,410 39,846,928 Net loss per share attributable to common stockholders, basic and diluted $ (2.43 ) $ (2.21 ) $ (2.30 )
Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share	The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Year Ended December 31, 2018 2017 2016 Stock options to purchase common stock 7,561,719 6,125,692 5,069,133 Unvested restricted stock units 226,900 356,778 115,500 Shares issuable under ESPP 49,495 — — 7,838,114 6,482,470 5,184,633

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments for Operating Leases	Future minimum lease payments for these operating leases as of December 31, 2018 are as follows: Year Ending December 31, 2019 $ 6,342 2020 6,120 2021 6,221 2022 6,372 2023 5,158 2024 and thereafter — Total $ 30,213

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Year Ended December 31, 2018 2017 2016 Federal statutory income tax rate (21.0 )% (34.0 )% (34.0 )% Research and development tax credits (7.2 ) (7.7 ) (8.8 ) State taxes, net of federal benefit (7.2 ) (4.8 ) (5.8 ) Stock-based compensation 2.0 1.2 (6.3 ) Revaluation of preferred stock warrant liability — 33.1 — Other 0.8 (0.3 ) 0.1 Change in deferred tax asset valuation allowance 32.6 12.5 54.8 Effective income tax rate — % — % — %
Schedule of Net Deferred Tax Assets	Net deferred tax assets as of December 31, 2018 and 2017 consisted of the following: December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 50,723 $ 32,997 Research and development tax credit carryforwards 32,522 25,405 Capitalized organization costs 275 306 Stock-based compensation expense 9,178 7,714 Charitable Contributions 11 8 Deferred Revenue 37,499 26,480 Accrued expenses 4,279 4,613 Capitalized research and development expenses 66 73 Total deferred tax assets $ 134,553 $ 97,596 Deferred tax liabilities: Depreciation and amortization (2,544 ) (3,470 ) Total deferred tax liabilities (2,544 ) (3,470 ) Valuation allowance $ (132,009 ) $ (94,126 ) Net deferred tax assets $ — $ —
Summary of Changes In Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2018, 2017 and 2016 related primarily to the increases in NOLs, research and development tax credit carryforwards, stock-based compensation and decrease of the deferred rate due to tax reform were as follows: Year Ended December 31, 2018 2017 2016 Valuation allowance at beginning of year $ (94,126 ) $ (82,994 ) $ (32,777 ) Decreases recorded as benefit to income tax provision — 29,546 — Increases recorded to income tax provision (37,883 ) (40,678 ) (50,217 ) Valuation allowance as of end of year $ (132,009 ) $ (94,126 ) $ (82,994 )

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Schedule of Quarterly Financial Information	The following table contains quarterly financial information for 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party (1) $ 3,766 $ 4,271 $ 8,684 $ 10,196 $ 26,917 Grant revenue 205 341 371 433 1,350 Total operating expenses 32,237 32,748 31,266 32,300 128,551 Loss from operations (28,266 ) (28,136 ) (22,211 ) (21,671 ) (100,284 ) Net loss (27,919 ) (27,787 ) (21,949 ) (21,287 ) (98,942 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.69 ) $ (0.68 ) $ (0.54 ) $ (0.52 ) $ (2.43 ) 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total Collaboration revenue - related party (1) $ 3,015 $ 3,014 $ 23,015 $ 3,056 $ 32,100 Total operating expenses 28,905 31,430 30,329 32,831 123,495 Loss from operations (25,890 ) (28,416 ) (7,314 ) (29,775 ) (91,395 ) Net loss (25,474 ) (28,018 ) (6,935 ) (28,953 ) (89,380 ) Net loss per share applicable to common stockholders - basic and diluted $ (0.63 ) $ (0.69 ) $ (0.17 ) $ (0.72 ) $ (2.21 ) (1) The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The Company’s reported results for 2018 reflect the application of ASC 606 guidance while its reported results for 2017 and 2016 were prepared under the guidance outlined in ASC 605. Refer to Note 9 for further details.

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated year									Oct. 31, 2010
Entity incorporated state name									State of Delaware
Accumulated deficit	$ 389,370				$ 263,571				$ 389,370	$ 263,571
Net loss	(21,287)	$ (21,949)	$ (27,787)	$ (27,919)	(28,953)	$ (6,935)	$ (28,018)	$ (25,474)	(98,942)	(89,380)	$ (91,579)
Cash used in operations									(62,854)	(75,523)	43,921
Cash and cash equivalents	$ 85,820				$ 36,088				$ 85,820	$ 36,088	$ 54,539	$ 73,933

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 01, 2019	Dec. 31, 2017
Accounting Policies [Line Items]
Maximum maturity days for cash equivalents	90 days
Restricted cash	$ 113,000		$ 113,000
Restricted investments	1,400,000		$ 1,400,000
Impairment losses on long lived assets	0
Revenue related to sales-based royalties	0
Revenue related to milestone payments	$ 0
Minimum percentage of likelihood of tax benefit being realized upon settlement	50.00%
ASU 2016-02 [Member] | Subsequent Event [Member]
Accounting Policies [Line Items]
Right-of-use assets		$ 14,000
Lease liabilities		$ 25,000
Laboratory Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset	5 years
Computer Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset	3 years
Furniture and Office Equipment [Member]
Accounting Policies [Line Items]
Useful life of asset	3 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 85,820	$ 36,088	$ 54,539	$ 73,933
Restricted cash	113	113	1,400	1,539
Total cash, cash equivalents and restricted cash as shown in the statement of cash flows	$ 85,933	$ 36,201	$ 55,939	$ 75,472

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Investments:
Investments			$ 113,894
Commercial Paper [Member]
Investments:
Investments			6,198
Certificates of Deposit [Member]
Investments:
Investments			8,916
Corporate Bonds [Member]
Investments:
Investments			58,865
Government Securities [Member]
Investments:
Investments			22,954
Treasury Bonds [Member]
Investments:
Investments			16,961
Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents		$ 39,982	25,964
Total assets		39,982	139,858
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Investments:
Investments			6,198
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Investments:
Investments			8,916
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Investments:
Investments			58,865
Fair Value, Measurements, Recurring [Member] | Government Securities [Member]
Investments:
Investments			22,954
Fair Value, Measurements, Recurring [Member] | Treasury Bonds [Member]
Investments:
Investments			16,961
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total assets			113,894
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Investments:
Investments			6,198
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Investments:
Investments			8,916
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Investments:
Investments			58,865
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Government Securities [Member]
Investments:
Investments			22,954
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Treasury Bonds [Member]
Investments:
Investments			16,961
Not Subject to Leveling [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents	[1]	39,982	25,964
Total assets	[1]	$ 39,982	$ 25,964
[1]	Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Restricted cash	$ 113	$ 113	$ 1,400	$ 1,539
Restricted investments	$ 1,400	$ 1,400

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Investments [Abstract]
Investments	$ 0	$ 113,895,000
Maximum maturity days for cash equivalents	90 days
Restricted investments	$ 1,400,000	$ 1,400,000

Investments - Schedule of Fair

Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 114,040
Gross Unrealized Loss	(146)
Fair Value	113,894
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,198
Fair Value	6,198
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,916
Fair Value	8,916
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,937
Gross Unrealized Loss	(72)
Fair Value	58,865
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,997
Gross Unrealized Loss	(43)
Fair Value	22,954
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,992
Gross Unrealized Loss	(31)
Fair Value	$ 16,961

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 46,595	$ 45,370
Less: Accumulated depreciation and amortization	(20,301)	(12,439)
Property and equipment, net	26,294	32,931
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,695	13,181
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,864	2,832
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,033	1,033
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,977	27,963
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 26	$ 361

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 7,862	$ 7,259	$ 4,205

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Development and clinical manufacturing costs	$ 7,046	$ 3,910
Payroll and payroll-related costs	5,020	4,962
Facility and other	3,141	3,641
Total accrued expenses and other current liabilities	$ 15,207	$ 12,513

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Jul. 01, 2015
Equity [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001	$ 0.001

Stockholders' Equity Common S_3

Stockholders' Equity Common Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Jul. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par value	$ 0.001	$ 0.001
Initial Public Offering ("IPO") [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized			200,000,000
Common stock, par value			$ 0.001

Stockholders' Equity Common S_4

Stockholders' Equity Common Stock - 2012 Stock Incentive plan - Additional Information (Detail) - 2012 Stock Incentive Plan [Member]	12 Months Ended
	Dec. 31, 2018shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan description	The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally granted stock-based awards with service conditions only (“service-based” awards).
Shares available for future grant	0
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price per share of common stock expressed as a percentage	100.00%
Stock option granted vesting period	4 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option term	10 years
Stock option granted vesting period	4 years
Stock option expiration period	10 years

Stockholders' Equity Common S_5

Stockholders' Equity Common Stock - 2015 Incentive Award Plan - Additional Information (Detail) - 2015 Incentive Award Plan [Member] - shares	Jun. 16, 2015	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares initially reserved for issuance	2,200,000
Percentage of common stock shares outstanding	4.00%
Stock incentive plan description		The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.
Shares available for future grant		1,353,580
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		4 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option granted vesting period		4 years
Stock option expiration period		10 years

Stockholders' Equity Common S_6

Stockholders' Equity Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 16, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 16,641	$ 17,362	$ 16,857
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock reserved for issuance	365,000	1,522,488
Shares issued under ESPP	400,000	33,332
Percentage of common stock shares outstanding	1.00%
Percentage of eligible earning contributed toward semi-annual purchase of common stock		15.00%
Offering period under the plan		6 months
Percentage of purchase price with respect to each offering period		85.00%
Risk-free interest rate		2.30%
Expected term (in years)		6 months
Expected volatility		60.20%
Expected dividend yield		0.00%
Stock-based compensation expense		$ 145	$ 0	$ 0

Stockholders' Equity Common S_7

Stockholders' Equity Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail) - Stock Option Valuation [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.39%	2.20%	1.54%
Expected term (in years)	6 years	6 years	6 years
Expected volatility	76.00%	80.90%	84.20%
Expected dividend yield	0.00%	0.00%	0.00%

Stockholders' Equity Common S_8

Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Number of Shares, Beginning Balance	6,125,692
Number of Shares, Granted	2,074,776
Number of Shares, Exercised	(212,240)
Number of Shares, Forfeited	(426,509)
Number of Shares, Ending Balance	7,561,719	6,125,692
Number of Shares, Options vested and expected to vest	7,561,719
Number of Shares, Options exercisable	4,386,848
Weighted Average Exercise Price, Beginning Balance	$ 13
Weighted Average Exercise Price, Granted	9.67
Weighted Average Exercise Price, Exercised	0.69
Weighted Average Exercise Price, Forfeited	15.99
Weighted Average Exercise Price, Ending Balance	12.26	$ 13
Weighted Average Exercise Price, Options vested and expected to vest	12.26
Weighted Average Exercise Price, Options exercisable	$ 12.56
Weighted Average Remaining Contractual Term, Outstanding	7 years 2 months 23 days	7 years 8 months 12 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	7 years 2 months 23 days
Weighted Average Remaining Contractual Term, Options exercisable	6 years 2 months 15 days
Aggregate Intrinsic Value, Outstanding	$ 4,958	$ 15,808
Aggregate Intrinsic Value, Options vested and expected to vest	4,958
Aggregate Intrinsic Value, Options exercisable	$ 4,958

Stockholders' Equity Common S_9

Stockholders' Equity Common Stock - Stock Options - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options	$ 6.53	$ 7.07	$ 18.45
Intrinsic Value, Stock options exercised	$ 1,801	$ 1,912	$ 34,871
Stock based compensation expense for stock options	$ 16,641	$ 17,362	$ 16,857
Outstanding unvested service-based stock options	193,282	45,000
Stock Option [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-Based stock options vested			60,000
Stock based compensation expense for stock options			$ 235
Grant date fair value of options			$ 3.92

Stockholders' Equity Common _10

Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) - Restricted Stock Units [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance | shares	356,778
Number of Shares, Granted | shares	30,900
Number of Shares, Forfeited | shares	(22,730)
Number of Shares, Vested | shares	(138,048)
Number of Shares, Unvested restricted stock units, Ending balance | shares	226,900
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance | $ / shares	$ 10.01
Weighted Average Grant Date Fair Value, Granted | $ / shares	7.50
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	9.87
Weighted Average Grant Date Fair Value, Vested | $ / shares	10.09
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance | $ / shares	$ 9.64

Stockholders' Equity Common _11

Stockholders' Equity Common Stock - Restricted Stock Units - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value, vested	$ 1,206	$ 445	$ 0

Stockholders' Equity Common _12

Stockholders' Equity Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 16,641	$ 17,362	$ 16,857
Research and development expenses [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	8,388	8,115	8,310
General and administrative expenses [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 8,253	$ 9,247	$ 8,547

Stockholders' Equity Common _13

Stockholders' Equity Common Stock - Stock Based Compensation - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Equity [Abstract]
Unrecognized stock- based compensation cost	$ 21,554
Weighted average period over which unrecognized compensation cost is expected to be recognized	2 years 2 months 8 days

Collaboration Revenue - Additio

Collaboration Revenue - Additional Information (Detail) - USD ($)		3 Months Ended									12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2018	Jan. 01, 2018	Feb. 29, 2016	Jan. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration/license fee										$ 120,000,000	$ 120,000,000
Period of performance identifiable deliverables											10 years
Total reported liabilities		$ 168,517,000				$ 128,823,000					$ 168,517,000	$ 128,823,000
Deferred revenue, net of current portion - related party		116,840,000				84,847,000					116,840,000	84,847,000
Deferred revenue - related party		20,419,000				12,079,000					20,419,000	12,079,000
Collaboration revenue - related party											26,917,000
Net Loss		(21,287,000)	$ (21,949,000)	$ (27,787,000)	$ (27,919,000)	(28,953,000)	$ (6,935,000)	$ (28,018,000)	$ (25,474,000)		(98,942,000)	(89,380,000)	$ (91,579,000)
Increase in deferred revenue											13,476,000	(11,888,000)	108,814,000
Collaboration revenue - related party											26,917,000	32,100,000	21,766,000
Collaboration Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party	[1]	10,196,000	8,684,000	$ 4,271,000	$ 3,766,000	3,056,000	$ 23,015,000	$ 3,014,000	$ 3,015,000		26,917,000	32,100,000
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		137,259,000				96,926,000					137,259,000	96,926,000				$ 120,000,000
Maximum development milestone payments to be received		285,000,000									285,000,000
Maximum regulatory payments to be received		375,000,000									375,000,000
Maximum amount to be received on achievement of certain commercial milestones		1,125,000,000									1,125,000,000
Upfront collaboration milestone payments receivable		20,000,000									20,000,000
Proceeds on achievement of development milestone												20,000,000	10,000,000
Transaction price allocated to remaining performance obligations		190,000,000									$ 190,000,000				$ 150,000,000
Transaction price of milestone payment to be received			20,000,000
Revenue information used to determine transaction price											The Company estimated the $20,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled.
Collaboration revenue - related party											$ 26,917,000	32,100,000	21,766,000
Deferred revenue		137,259,000				96,926,000					137,259,000	96,926,000
Nestlé Health Science [Member] | Collaboration Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party											26,917,000	32,100,000	21,766,000
Nestlé Health Science [Member] | Phase 2 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable																	$ 20,000,000
Transaction price of milestone payment to be received			$ 20,000,000
Revenue recognition cumulative catch-up adjustment											5,020,000
Nestlé Health Science [Member] | Phase 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable																	$ 20,000,000
Transaction price of milestone payment to be received		20,000,000
Revenue recognition cumulative catch-up adjustment											5,517,000
Nestlé Health Science [Member] | Phase 3 [Member] | Collaboration Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party												20,000,000	10,000,000
Nestlé Health Science [Member] | Phase 2b [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable														$ 40,000,000
Proceeds on achievement of development milestone											40,000,000
Nestlé Health Science [Member] | Upfront License Fee [Member] | Collaboration Revenue - Related Party [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party												12,100,000	$ 11,766,000
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, net of current portion - related party															106,365,000
Deferred revenue - related party															17,418,000
Upfront cash payment		137,259,000				$ 123,783,000					137,259,000	$ 123,783,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total reported liabilities		52,137,000									52,137,000
Deferred revenue, net of current portion - related party		43,836,000									43,836,000				21,518,000
Deferred revenue - related party		$ 8,301,000									8,301,000				$ 5,339,000
Net Loss											25,280,000
Increase in deferred revenue											25,280,000
Difference Between Proforma Value and Reported Value [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party											$ 25,280,000
[1]	The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The Company’s reported results for 2018 reflect the application of ASC 606 guidance while its reported results for 2017 and 2016 were prepared under the guidance outlined in ASC 605. Refer to Note 9 for further details.

Collaboration Revenue - Adjustm

Collaboration Revenue - Adjustments to Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	$ 20,419		$ 12,079
Deferred revenue, net of current portion - related party	116,840		84,847
Accumulated deficit	(389,370)		$ (263,571)
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party		$ 17,418
Deferred revenue, net of current portion - related party		106,365
Accumulated deficit		(290,428)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	8,301	5,339
Deferred revenue, net of current portion - related party	$ 43,836	21,518
Accumulated deficit		$ (26,857)

Collaboration Revenue - Compari

Collaboration Revenue - Comparison of Reported Consolidated Balance Sheet to Pro-forma Amounts (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	$ 20,419		$ 12,079
Deferred revenue, net of current portion - related party	116,840		84,847
Accumulated deficit	(389,370)		$ (263,571)
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party		$ 17,418
Deferred revenue, net of current portion - related party		106,365
Accumulated deficit		$ (290,428)
Previous Accounting in Effect [Member] | ASU 2014-09 [Member] | Pro Forma [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	12,118
Deferred revenue, net of current portion - related party	73,004
Accumulated deficit	$ (337,233)

Collaboration Revenue - Compa_2

Collaboration Revenue - Comparison of Reported Consolidated Statements of Operations and Comprehensive Loss to Pro-forma Amounts (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party									$ 26,917
Loss from operations	$ (21,671)	$ (22,211)	$ (28,136)	$ (28,266)	$ (29,775)	$ (7,314)	$ (28,416)	$ (25,890)	(100,284)	$ (91,395)	$ (92,839)
Net loss	$ (21,287)	$ (21,949)	$ (27,787)	$ (27,919)	$ (28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (98,942)	$ (89,380)	$ (91,579)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.52)	$ (0.54)	$ (0.68)	$ (0.69)	$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (2.43)	$ (2.21)	$ (2.30)
Previous Accounting in Effect [Member] | ASU 2014-09 [Member] | Pro Forma [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party									$ 52,197
Loss from operations									(75,004)
Net loss									$ (73,662)
Net loss per share attributable to common stockholders, basic and diluted									$ (1.81)

Collaboration Revenue - Compa_3

Collaboration Revenue - Comparison of Reported Consolidated Statements of Cash Flows to Pro-forma Amounts (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net loss	$ (21,287)	$ (21,949)	$ (27,787)	$ (27,919)	$ (28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (98,942)	$ (89,380)	$ (91,579)
Deferred revenue									13,476	$ (11,888)	$ 108,814
Previous Accounting in Effect [Member] | ASU 2014-09 [Member] | Pro Forma [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net loss									(73,662)
Deferred revenue									$ (11,804)

Collaboration Revenue - Changes

Collaboration Revenue - Changes in Contract Liabilities (Detail) - ASU 2014-09 [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Contract liabilities:
Balance at Beginning of Period	$ 123,783
Additions	40,393
Deductions	(26,917)
Balance at End of Period	$ 137,259

Collaboration Revenue - Schedul

Collaboration Revenue - Schedule of Revenue Recognized (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
ASU 2014-09 [Member]
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 15,774

Net Loss per Share - Basic and

Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss attributable to common stockholders									$ (98,942)	$ (89,380)	$ (91,579)
Denominator:
Weighted average common shares outstanding, basic and diluted									40,743,492	40,449,410	39,846,928
Net loss per share attributable to common stockholders, basic and diluted	$ (0.52)	$ (0.54)	$ (0.68)	$ (0.69)	$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (2.43)	$ (2.21)	$ (2.30)

Net Loss per Share - Summary of

Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	7,838,114	6,482,470	5,184,633
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	7,561,719	6,125,692	5,069,133
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	226,900	356,778	115,500
Shares Issuable under ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	49,495

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Nov. 11, 2015USD ($)ft²	Apr. 01, 2015	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)
Other Commitments [Line Items]
Operating lease agreement expiration date			Jun. 30, 2016
Operating lease termination write off of deferred rent benefit			$ 136,000
Outstanding security deposit						$ 119,000
Indemnification Agreement [Member]
Other Commitments [Line Items]
Indemnification obligations accrued			$ 0	$ 0
BMR Sidney Research Campus LLC [Member]
Other Commitments [Line Items]
Non-cancelable property lease, description			On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016
Square feet of property subject to non-cancelable lease | ft²	83,396
Lease commencement period			2016-03
Lease expiration period			2023-11
Cost of tenant improvements	$ 12,509,000
Lease incentive obligation			$ 8,544,000
Amortized of lease incentive obligation			1,768,000
Office and Laboratory Space [Member]
Other Commitments [Line Items]
Operating lease agreement expiration date		Apr. 30, 2020
Rental expense related to office and laboratory space			$ 4,377,000	$ 4,458,000	$ 3,826,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary Of Future Minimum Lease Payments For Operating Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 6,342
2,020	6,120
2,021	6,221
2,022	6,372
2,023	5,158
Total	$ 30,213

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Income tax benefit	$ 0	$ 0	$ 0
TCJA adjustment to provisional amounts previously recorded	0
Unrecognized tax benefits	0	$ 0	$ 0
Massachusetts Life Sciences Center [Member]
Income Tax Disclosure [Line Items]
Tax incentives awarded	170,000
Tax incentive program period			5 years
Massachusetts [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 185,927,000
Net operating loss carry forwards, expiration year, start	2,035
After 2017 [Member] | Maximum [Member]
Income Tax Disclosure [Line Items]
Taxable income on NOL	80.00%
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 185,583,000
Federal [Member] | Before 2018 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 119,781,000
Net operating loss carry forwards, expiration year, start	2,035
Federal [Member] | After 2017 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 65,801,000
Federal [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 28,638,000
Tax credit carryforwards, expiration year, start	2,031
Federal [Member] | Orphan Drug Credit Carryforwards [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 17,552,000
State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	185,927,000
State [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 4,916,000
Tax credit carryforwards, expiration year, start	2,028

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(21.00%)	(34.00%)	(34.00%)
Research and development tax credits	(7.20%)	(7.70%)	(8.80%)
State taxes, net of federal benefit	(7.20%)	(4.80%)	(5.80%)
Stock-based compensation	2.00%	1.20%	(6.30%)
Revaluation of preferred stock warrant liability		33.10%
Other	0.80%	(0.30%)	0.10%
Change in deferred tax asset valuation allowance	32.60%	12.50%	54.80%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 50,723	$ 32,997
Research and development tax credit carryforwards	32,522	25,405
Capitalized organization costs	275	306
Stock-based compensation expense	9,178	7,714
Charitable Contributions	11	8
Deferred Revenue	37,499	26,480
Accrued expenses	4,279	4,613
Capitalized research and development expenses	66	73
Total deferred tax assets	134,553	97,596
Deferred tax liabilities:
Depreciation and amortization	(2,544)	(3,470)
Total deferred tax liabilities	(2,544)	(3,470)
Valuation allowance	$ (132,009)	$ (94,126)	$ (82,994)	$ (32,777)

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Abstract]
Valuation allowance at beginning of year	$ (94,126)	$ (82,994)	$ (32,777)
Decreases recorded as benefit to income tax provision		29,546
Increases recorded to income tax provision	(37,883)	(40,678)	(50,217)
Valuation allowance as of end of year	$ (132,009)	$ (94,126)	$ (82,994)

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Collaboration revenue - related party										$ 26,917	$ 32,100	$ 21,766
Grant revenue		$ 433	$ 371	$ 341	$ 205					1,350
Total operating expenses		32,300	31,266	32,748	32,237	$ 32,831	$ 30,329	$ 31,430	$ 28,905	128,551	123,495	114,605
Loss from operations		(21,671)	(22,211)	(28,136)	(28,266)	(29,775)	(7,314)	(28,416)	(25,890)	(100,284)	(91,395)	(92,839)
Net loss		$ (21,287)	$ (21,949)	$ (27,787)	$ (27,919)	$ (28,953)	$ (6,935)	$ (28,018)	$ (25,474)	$ (98,942)	$ (89,380)	$ (91,579)
Net loss per share attributable to common stockholders, basic and diluted		$ (0.52)	$ (0.54)	$ (0.68)	$ (0.69)	$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (2.43)	$ (2.21)	$ (2.30)
Collaboration Revenue - Related Party [Member]
Quarterly Financial Information [Line Items]
Collaboration revenue - related party	[1]	$ 10,196	$ 8,684	$ 4,271	$ 3,766	$ 3,056	$ 23,015	$ 3,014	$ 3,015	$ 26,917	$ 32,100
[1]	The Company adopted ASC 606 on January 1, 2018, using the modified retrospective method for all contracts not completed as of the date of adoption. The Company’s reported results for 2018 reflect the application of ASC 606 guidance while its reported results for 2017 and 2016 were prepared under the guidance outlined in ASC 605. Refer to Note 9 for further details.

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 26,917,000	$ 32,100,000	$ 21,766,000
Flagship Pioneering [Member] | Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0	0	17,000
Due to related party	0	0
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0
Collaboration revenue - related party	26,917,000	32,100,000	$ 21,766,000
Deferred revenue	137,259,000	$ 96,926,000		$ 120,000,000
Due from related party	$ 0

401(k) Savings Plan - Additiona

401(k) Savings Plan - Additional Information (Detail) - USD ($) $ in Thousands	Jan. 01, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Employer match of employee contributions	50.00%
Employee deferral subject to employer match	6.00%
Employer contribution expenses		$ 604	$ 484	$ 457

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - Subsequent Event [Member]	Feb. 07, 2019USD ($)Employee
Subsequent Event [Line Items]
Reduction in headcount of employees | Employee	30
One-Time Costs [Member] | Minimum [Member]
Subsequent Event [Line Items]
Expected employee termination benefits	$ 1,500,000
One-Time Costs [Member] | Maximum [Member]
Subsequent Event [Line Items]
Expected employee termination benefits	$ 2,000,000