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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	0001609809
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity File Number	001-37465
Entity Tax Identification Number	274326290
Entity Address, Address Line One	200 Sidney Street - 4th Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	945-9626
Entity Common Stock Shares Outstanding		69,913,410

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 102,230	$ 85,820
Prepaid expenses and other current assets	5,160	6,845
Accounts receivable	1,604
Total current assets	108,994	92,665
Property and equipment, net	22,945	26,294
Operating lease assets	12,640
Restricted investments	1,400	1,400
Restricted cash	114	113
Total assets	146,093	120,472
Current liabilities:
Accounts payable	4,764	6,415
Accrued expenses and other current liabilities	10,589	15,207
Operating lease liabilities	4,449
Deferred revenue - related party	20,582	20,419
Deferred revenue	2,689
Total current liabilities	43,073	42,041
Operating lease liabilities, net of current portion	17,969
Lease incentive obligation, net of current portion		6,776
Deferred rent		2,216
Deferred revenue, net of current portion - related party	99,608	116,840
Deferred revenue, net of current portion	2,752
Other long-term liabilities	659	644
Total liabilities	164,061	168,517
Commitments and contingencies (Note 11)
Stockholders’ deficit:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at June 30, 2019 and December 31, 2018; no shares issued and outstanding at June 30, 2019 and December 31, 2018
Common stock, $0.001 par value; 200,000,000 shares authorized at June 30, 2019 and December 31, 2018; 69,913,410 and 40,936,735 shares issued and outstanding at June 30, 2019 and December 31, 2018, respectively	70	41
Additional paid-in capital	406,424	341,284
Accumulated deficit	(424,462)	(389,370)
Total stockholders’ deficit	(17,968)	(48,045)
Total liabilities and stockholders’ deficit	$ 146,093	$ 120,472

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
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	69,913,410	40,936,735
Common stock, shares outstanding	69,913,410	40,936,735

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue:
Collaboration revenue - related party	$ 10,454,000	$ 4,271,000	$ 17,069,000	$ 8,037,000
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue	$ 260,000	$ 341,000	$ 706,000	$ 546,000
Revenue	1,817,000		2,077,000
Total revenue	12,531,000	4,612,000	19,852,000	8,583,000
Operating expenses:
Research and development expenses	17,905,000	24,053,000	40,792,000	47,513,000
General and administrative expenses	5,574,000	8,695,000	13,069,000	17,472,000
Restructuring expenses	0		1,492,000
Total operating expenses	23,479,000	32,748,000	55,353,000	64,985,000
Loss from operations	(10,948,000)	(28,136,000)	(35,501,000)	(56,402,000)
Other income (expense):
Interest income (expense), net	189,000	349,000	409,000	696,000
Total other income (expense), net	189,000	349,000	409,000	696,000
Net loss	$ (10,759,000)	$ (27,787,000)	$ (35,092,000)	$ (55,706,000)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.24)	$ (0.68)	$ (0.81)	$ (1.37)
Weighted average common shares outstanding, basic and diluted	45,140,830	40,661,464	43,095,686	40,645,040
Net loss	$ (10,759,000)	$ (27,787,000)	$ (35,092,000)	$ (55,706,000)
Other comprehensive income:
Unrealized gain on investments, net of tax of $0		77,000		117,000
Total other comprehensive income		77,000		117,000
Comprehensive loss	$ (10,759,000)	$ (27,710,000)	$ (35,092,000)	$ (55,589,000)

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Unrealized gain on investment, tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2017	$ 60,699	$ 40	$ 324,376	$ (263,571)	$ (146)
Beginning balance, shares at Dec. 31, 2017		40,571,015
Issuance of common stock upon exercise of stock options, value	32		32
Issuance of common stock upon exercise of stock options, shares		48,053
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		51,500
Repurchase of common stock for employee tax withholdings, value	(197)		(197)
Repurchase of common stock for employee tax withholdings, shares		(17,900)
Stock-based compensation expense	4,236		4,236
Unrealized gain on investments	40				40
Adoption of new revenue standard (ASC 606)	(26,857)			(26,857)
Net loss	(27,919)			(27,919)
Ending balance at Mar. 31, 2018	10,034	$ 40	328,447	(318,347)	(106)
Ending balance, shares at Mar. 31, 2018		40,652,668
Beginning balance at Dec. 31, 2017	60,699	$ 40	324,376	(263,571)	(146)
Beginning balance, shares at Dec. 31, 2017		40,571,015
Net loss	(55,706)
Ending balance at Jun. 30, 2018	(13,251)	$ 40	332,872	(346,134)	(29)
Ending balance, shares at Jun. 30, 2018		40,754,681
Beginning balance at Mar. 31, 2018	10,034	$ 40	328,447	(318,347)	(106)
Beginning balance, shares at Mar. 31, 2018		40,652,668
Issuance of common stock upon exercise of stock options, value	64		64
Issuance of common stock upon exercise of stock options, shares		92,013
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	27		27
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		10,000
Stock-based compensation expense	4,334		4,334
Unrealized gain on investments	77				77
Net loss	(27,787)			(27,787)
Ending balance at Jun. 30, 2018	(13,251)	$ 40	332,872	(346,134)	$ (29)
Ending balance, shares at Jun. 30, 2018		40,754,681
Beginning balance at Dec. 31, 2018	$ (48,045)	$ 41	341,284	(389,370)
Beginning balance, shares at Dec. 31, 2018	40,936,735	40,936,735
Issuance of common stock upon exercise of stock options, value	$ 120		120
Issuance of common stock upon exercise of stock options, shares		38,125
Issuance of common stock upon vesting of RSUs, net of tax withholdings, value	153		153
Issuance of common stock upon vesting of RSUs, net of tax withholdings, shares		73,500
Issuance of common stock under ESPP plan, value	207		207
Issuance of common stock under ESPP plan, shares		46,472
Stock-based compensation expense	2,065		2,065
Net loss	(24,333)			(24,333)
Ending balance at Mar. 31, 2019	(69,833)	$ 41	343,829	(413,703)
Ending balance, shares at Mar. 31, 2019		41,094,832
Beginning balance at Dec. 31, 2018	$ (48,045)	$ 41	341,284	(389,370)
Beginning balance, shares at Dec. 31, 2018	40,936,735	40,936,735
Issuance of common stock upon exercise of stock options, shares	38,125
Net loss	$ (35,092)
Ending balance at Jun. 30, 2019	$ (17,968)	$ 70	406,424	(424,462)
Ending balance, shares at Jun. 30, 2019	69,913,410	69,913,410
Beginning balance at Mar. 31, 2019	$ (69,833)	$ 41	343,829	(413,703)
Beginning balance, shares at Mar. 31, 2019		41,094,832
Issuance of common stock from public offering, net of commissions, underwriting, value	60,527	$ 29	60,498
Issuance of common stock from public offering, net of commissions, underwriting, shares		28,818,578
Stock-based compensation expense	2,097		2,097
Net loss	(10,759)			(10,759)
Ending balance at Jun. 30, 2019	$ (17,968)	$ 70	$ 406,424	$ (424,462)
Ending balance, shares at Jun. 30, 2019	69,913,410	69,913,410

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (35,092)	$ (55,706)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,162	8,570
Depreciation and amortization expense	3,927	3,895
Non-cash operating lease cost	1,097
Accretion of discount on investments		(132)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,685	(376)
Accounts receivable	(1,604)
Deferred revenue	(11,628)	(7,863)
Accounts payable	(1,645)	(1,537)
Operating lease liabilities	(2,079)
Accrued expenses and other current and long-term liabilities	(3,004)	712
Net cash (used in) operating activities	(44,181)	(52,437)
Cash flows from investing activities:
Purchases of property and equipment	(647)	(1,655)
Purchases of investments		(15,456)
Sales and maturities of investments		91,495
Net cash (used in) provided by investing activities	(647)	74,384
Cash flows from financing activities:
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	60,759
Proceeds from exercise of stock options	120	96
Proceeds from issuance of common stock and restricted common stock	153	26
Payments of employee tax obligations related to vesting of restricted stock units		(197)
Issuance of common stock under ESPP plan	207
Net cash provided by (used in) financing activities	61,239	(75)
Net increase in cash and cash equivalents	16,411	21,872
Cash, cash equivalents and restricted cash at beginning of period	85,933	37,601
Cash, cash equivalents and restricted cash at end of period	102,344	59,473
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	88	$ 90
Issuance costs from public offering included in accounts payable and accrued expenses	$ 232

Nature of the Business and Basis of Presentation

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Nature of the Business and Basis of Presentation

1.	Nature of the Business and Basis of Presentation

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 treat disease by restoring the function of a dysbiotic microbiome. The Company is developing SER-287 to treat ulcerative colitis (“UC”), a form of inflammatory bowel disease (“IBD”). SER-109, is designed to reduce recurrences of Clostridium difficile, or C. difficile infection (“CDI”), a debilitating infection of the colon, in patients who have received antibiotic therapy for recurrent CDI by treating the dysbiosis of the colonic microbiome, which, if approved by the U.S. Food and Drug Administration (“FDA”), could be a first-in-field oral microbiome drug. In addition, using its microbiome therapeutics platform, the Company is developing product candidates to treat diseases where the microbiome is implicated, including SER-301, a rationally designed, fermented IBD candidate, and SER-401, a microbiome therapeutic candidate for use with checkpoint inhibitors in patients with metastatic melanoma. Supporting the Company’s research and development efforts are its deep capabilities related to microbiome therapeutic drug discovery, manufacturing, quality, and clinical development. The Company believes that these capabilities provide it with important competitive advantages related to the advancement of this novel treatment modality.

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 pre-clinical 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, or maintained, that any product candidates 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.

In February 2019, the Company implemented corporate changes to focus its resources on advancing its clinical-stage therapeutic candidates. As a result, the Company now intends to concentrate on completing the SER-287 Phase 2b study in mild-to-moderate UC, obtaining results from the ongoing SER-109 Phase 3 study for recurrent CDI, 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, and advancing SER-301 into clinical development. In connection with the prioritization of these therapeutic candidates, the Company made changes to its management team and reduced headcount by approximately 30 percent.

On June 18, 2019, the Company completed an underwritten public offering, in which the Company sold 26,666,667 shares of its common stock at a price to the public of $2.25 per share. The aggregate net proceeds received by the Company from the offering were approximately $55,976, after deducting underwriting discounts and commissions and offering expenses payable by the Company. In addition, the Company granted the underwriters a 30-day option to purchase up to an additional 2,666,666 shares of common stock at the public offering price, less underwriting discounts and commissions. On June 21, 2019, the Company sold an additional 2,151,911 shares of its common stock at a price to the public of $2.25 per share. The aggregate net proceeds received by the Company were approximately $4,551, after deducting underwriting discounts and commissions and offering expenses payable by the Company.

The Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the condensed consolidated financial statements are issued. The Company has experienced negative cash flows and had an accumulated deficit of $424,462 and $389,370 as of June 30, 2019 and December 31, 2018, respectively. For the six months ended June 30, 2019, the Company incurred a loss of $35,092 and used $44,181 of cash in operations. The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. The Company expects that its cash and cash equivalents at June 30, 2019 of $102,230 will be sufficient to fund its operating expenses and capital expenditure requirements for at least the next 12 months from issuance of the financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to finance its operations.

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.

Unaudited Interim Financial Information

The accompanying unaudited condensed consolidated financial statements as of June 30, 2019 and for the three and six months ended June 30, 2019 and 2018 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s annual report on Form 10-K for the fiscal year ended December 31, 2018, which was filed with the SEC on March 6, 2019 (the “Annual Report”).

The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited consolidated financial statements. The condensed consolidated balance sheet at December 31, 2018 was derived from audited annual financial statements, but does not contain all of the footnote disclosures from the annual financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments which are necessary for a fair presentation of the Company’s financial position, results of operations, and cash flows for the periods presented. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2019.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

The significant accounting policies and estimates used in preparation of the condensed consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Annual Report. There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2019 except for the adoption of the new lease accounting standard discussed in Note 9, Leases.

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 revenue and 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.

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 restricted stock units granted by the Company entitle the holder of such awards to ordinary cash dividends paid to substantially all holders of the Company’s common stock, as if such shares were outstanding common shares at the time of the dividend. The dividends are paid in cash or shares of common stock when the applicable restricted stock unit vests. However, the unvested restricted stock units 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.

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:

	Three and Six Months Ended June 30,
	2019		2018
Stock options to purchase common stock		8,628,459		7,481,478
Unvested restricted stock units		130,900		289,248
Shares issuable under ESPP		37,537		—
Total common stock equivalents		8,796,896		7,770,726

Leases

The Company evaluates arrangements at inception to determine if an arrangement is or contains a lease. Operating lease assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease assets and liabilities are recognized at the commencement date of the lease based upon the present value of lease payments over the lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. The Company uses an incremental borrowing rate that the Company would expect to incur for a fully collateralized loan over a similar term under similar economic conditions to determine the present value of the lease payments. The lease payments used to determine the Company’s operating lease assets may include lease incentives and stated rent increases and are recognized in the Company’s operating lease assets in the Company’s condensed consolidated balance sheets. Lease expense for operating lease payments is recognized on a straight-line basis over the term of the lease. Variable lease payments are recognized as the associated obligation is incurred.

Restructuring

Restructuring costs are comprised of severance costs related to workforce reductions. The Company recognizes restructuring charges when the liability is incurred. Employee termination benefits are accrued at the date management has committed to a plan of termination and employees have been notified of their termination dates and expected severance payments.

Recently Issued Accounting Standards

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), (“Topic 842”), which establishes principles that lessees and lessors shall apply to report useful information to users of financial statements about the amount, timing and uncertainty of cash flows arising from a lease. The most notable change is lessees recognizing an asset and liability on their balance sheet for operating leases. In 2018, the FASB issued ASU 2018-01, and ASU 2018-11, which collectively adds two practical expedients, provides a second modified retrospective transition method which does not require retrospective adjustment of prior periods, and provides certain narrow scope improvements to the new lease guidance. ASU 2016-02 and the amending ASUs are effective for the Company for annual periods beginning after December 15, 2018 and interim periods therein, with early adoption permitted.

The Company adopted the new guidance as of January 1, 2019 using the modified retrospective transition approach with no restatement of prior periods or cumulative adjustment to accumulated deficit. Upon adoption, the Company elected the package of transition practical expedients, which allowed the Company to carry forward prior conclusions related to whether any expired or existing contracts are or contain leases, the lease classification for any expired or existing leases and initial direct costs for existing leases. The Company also made an accounting policy election not to recognize leases with an initial term of 12 months or less within its condensed consolidated balance sheets and to recognize those lease payments on a straight-line basis in its condensed consolidated statements of operations and comprehensive loss over the lease term. Upon adoption of the new leasing standards, the Company recognized an operating lease asset of approximately $13,737 and a corresponding operating lease liability of approximately $24,497, which are included in the Company’s condensed consolidated balance sheet. The adoption of the new leasing standards did not have any impact on the Company’s condensed consolidated statements of operations and comprehensive loss. The impact to the Condensed Consolidated Balance Sheets for the opening balances is as follows (in thousands):

	December 31, 2018		Impact of adoption of ASC 842			January 1, 2019
Operating lease assets	$	—	$	13,737		$	13,737
Accrued expenses and other current liabilities		15,207		(1,768	)		13,439
Operating lease liabilities		—		4,285			4,285
Lease incentive obligation, net of current portion		6,776		(6,776	)		—
Deferred rent		2,216		(2,216	)		—
Operating lease liabilities, net of current portion		—		20,212			20,212

In June 2018, the FASB issued ASU 2018-07, “Compensation – Stock Compensation (Topic 718)” (“ASU 2018-07”). ASU 2018-07 simplifies the accounting for nonemployee share-based payment transactions. This ASU is effective for public entities for interim and annual reporting periods beginning after December 15, 2018. The Company adopted the ASU effective January 1, 2019, the impact of adoption of this standard was immaterial to the Company’s condensed consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). This standard eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of its disclosure framework project. ASU 2018-13 is effective for annual reporting periods beginning after December 15, 2019 and interim periods within those annual periods and early adoption is permitted. The Company is currently evaluating the impact of its adoption of ASU 2018-13 on its condensed consolidated financial statements.

In November 2018 the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. This standard makes targeted improvements for collaborative arrangements as follows:

•

Clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606, Revenue from Contracts with Customers, when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements;

•

Adds unit-of-account guidance to ASC 808, Collaborative Arrangements, to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606; and

•

Requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting that transaction together with revenue recognized under ASC 606 is precluded if the collaborative arrangement participant is not a customer.

This standard will be effective on January 1, 2020; however, early adoption is permitted. A retrospective transition approach is required for either all contracts or only for contracts that are not completed at the date of initial application of ASC 606, with a cumulative adjustment to opening retained earnings. The Company is currently evaluating the potential impact that this standard may have on its consolidated financial position and results of operations.

Fair Value Measurements

6 Months Ended

Jun. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.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.

As of June 30, 2019 and December 31, 2018, the Company’s cash equivalents consisted of money market funds of $20,393 and $39,982, respectively, with original maturities of less than 90 days from the date of purchase. The money market funds represent open-ended mutual funds with published daily net asset values at which investors can freely subscribe to or redeem from the funds and is classified as Level 1 in the fair value hierarchy. As of June 30, 2019 and December 31, 2018, the Company held a restricted investment of $1,400 and $1,400, respectively, which represent a certificate of deposit that is classified as Level 2 in the fair value hierarchy. There were no liabilities measured at fair value on a recurring basis as of June 30, 2019 or December 31, 2018.

The carrying values of the Company’s cash, other current assets, accounts receivable, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities.

- Definition

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

+ References + Details

Investments

6 Months Ended

Jun. 30, 2019

Investments Debt And Equity Securities [Abstract]

Investments

4.	Investments

As of June 30, 2019 and December 31, 2018 the Company had restricted investments of $1,400 which consisted of a certificate of deposit as a security deposit on its building lease at 200 Sidney Street, Cambridge, Massachusetts. The Company’s restricted investments are carried at amortized cost, which approximates fair value.

Property and Equipment, Net

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5.	Property and Equipment, Net

Property and equipment, net consisted of the following:

	June 30, 2019			December 31, 2018
Laboratory equipment	$	15,101		$	14,695
Computer equipment		2,874			2,864
Furniture and office equipment		1,033			1,033
Leasehold improvements		27,977			27,977
Construction in progress		26			26
		47,011			46,595
Less: Accumulated depreciation and amortization	$	(24,066	)	$	(20,301	)
	$	22,945		$	26,294

Depreciation and amortization expense was $1,921, $3,927, $1,954, and $3,895 for the three and six months ended June 30, 2019 and 2018, respectively.

Accrued Expenses and Other Current Liabilities

6 Months Ended

Jun. 30, 2019

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:

	June 30, 2019		December 31, 2018
Development and manufacturing costs	$	5,626	$	7,046
Payroll and payroll-related costs		3,516		5,020
Facility and other		1,447		3,141
	$	10,589	$	15,207

Stockholders' Deficit Common Stock

6 Months Ended

Jun. 30, 2019

Equity [Abstract]

Stockholders' Deficit Common Stock

7.	Stockholders’ Deficit Common Stock

Stock Options

The following table summarizes the Company’s stock option activity since 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, 2018		7,561,719		$	12.26	$	7.23	$	4,958
Granted		2,305,350			5.95
Exercised		(38,125	)		3.14
Forfeited		(1,200,485	)		12.08
Outstanding as of June 30, 2019		8,628,459		$	10.64	$	7.23	$	3,190
Options vested and expected to vest as of June 30, 2019		8,628,459			10.64		7.23		3,190
Options exercisable as of June 30, 2019		4,812,997		$	12.50		5.82	$	3,138

The weighted average grant-date fair value of stock options granted during the three and six months ended June 30, 2019 and 2018 was $2.38, $4.34, $6.89 and $5.82 per share, respectively.

During the six months ended June 30, 2019, the Company granted performance-based stock options to employees for the purchase of an aggregate of 1.1 million shares of common stock with a grant date fair value of $4.58 per share. These stock options are exercisable only upon achievement of specified performance targets. As of June 30, 2019, none of these options were exercisable because none of the specified performance targets had been achieved. Because achievement of the specified performance targets was not deemed probable as of June 30, 2019, the Company did not record any expense for these stock options from the dates of issuance through June 30, 2019.

Restricted Stock Units

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock unit activity since December 31, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2018		226,900		$	9.64
Granted		—			—
Forfeited		(22,500	)		8.77
Vested		(73,500	)		9.98
Unvested restricted stock units as of June 30, 2019		130,900		$	9.60

Stock-based Compensation Expense

The Company recorded stock-based compensation expense related to stock options, restricted stock units and the Company’s Employee Stock Purchase Plan (“ESPP”) in the following expense categories of its condensed consolidated statements of operations and comprehensive loss:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development expenses	$	1,093	$	2,262	$	2,541	$	4,190
General and administrative expenses		1,004		2,072		1,621		4,380
	$	2,097	$	4,334	$	4,162	$	8,570

Employee Stock Purchase Plan

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 recorded an immaterial amount of stock-based compensation expense under the ESPP for the three and six months ended June 30, 2019.

The total number of available ESPP shares is increased annually, which began in 2016 and will end in 2025. The ESPP allows for share replenishment equal to the lesser of (i) 400,000 shares and (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the preceding calendar year, or an amount determined by the board of directors. As of June 30, 2019, a total of 1.9 million shares were reserved and available for issuance under the ESPP.

Collaboration Revenue

6 Months Ended

Jun. 30, 2019

Revenue From Contract With Customer [Abstract]

Collaboration Revenue

8.	Collaboration Revenue

NHS Collaboration Agreement

Summary of Agreement

In January 2016, the Company entered into a collaboration and license agreement with NHS (“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. Pursuant to the letter agreement, the Company and NHS agreed that following initiation of the SER-287 Phase 2b study, the Company would be entitled to receive $40,000 in milestone payments from NHS, which represent the milestone payments due to the Company for the initiation of a SER-287 Phase 2 study and a Phase 3 study. 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—Revenue From Contracts with Customers (“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. 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. The transaction price as of June 30, 2019 was approximately $190,000.

In April 2019, the Company, with the approval of the Seres/NHS Joint Steering Committee, as provided for in the License Agreement, modified the SER-109 clinical trial. As a result of this modification, the Company and NHS agreed, and informed the FDA, that the target study enrollment would be reduced from 320 subjects to 188 subjects. This modification to the SER-109 clinical trial 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 modification reduced the total estimated costs in the Company’s cost-to-cost model for the License Agreement and resulted in the Company recognizing $6,830 of collaboration revenue – related party in the three and six months ended June 30, 2019.

During the three and six months ended June 30, 2019 and 2018, using the cost-to-cost method, which best depicts the transfer of control to the customer, the Company recognized $10,454, $17,069, $4,271 and $8,037 of Collaboration revenue – related party, respectively.

As of June 30, 2019 and December 31, 2018, there was $120,190, and $137,259, respectively, of deferred revenue related to the unsatisfied portion of the performance obligation under the License Agreement. As of June 30, 2019, 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 12 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 condensed consolidated statements of operations and comprehensive loss.

AstraZeneca Research Collaboration and Option Agreement

Summary of the Agreement

In March 2019, the Company entered into a Research Collaboration and Option Agreement (the “Research Agreement”) with MedImmune, LLC, a wholly owned subsidiary of AstraZeneca Inc. (“AstraZeneca”), to advance the mechanistic understanding of the microbiome in augmenting the efficacy of cancer immunotherapy. Under the Research Agreement, the Company and AstraZeneca will conduct certain research and development activities as set forth on a research plan focused on the role of the microbiome in certain cancers and cancer immunotherapies, including furthering the research program for SER-401, in combination with AstraZeneca compounds targeting various cancers.

Pursuant to the Research Agreement, the Company agreed not to conduct research or development on any microbiome products specifically designed by the Company during the term of the Research Agreement for the treatment of cancer (“Microbiome Oncology Products”), with or on behalf of any third party without the prior approval of the joint steering committee for the Research Agreement for at least three years after the effective date (the “Exclusivity Period”). Additionally, AstraZeneca will pay to the Company a total of $20,000 in three equal installments, the first of which the Company received in April 2019, and the second and third of which become due on January 2, 2020 and January 4, 2021, respectively. Such payments are payable even if the Research Agreement is terminated in accordance with its terms, unless the Research Agreement is terminated by AstraZeneca for the Company’s uncured material breach. Additionally, AstraZeneca will bear its costs of conducting activities under the research plan and will reimburse the Company for all activities performed under the research plan based on actual full-time employee (“FTE”) time and certain third-party costs incurred by the Company in connection therewith.

Under the Research Agreement, the Company granted to AstraZeneca an exclusive option to negotiate a worldwide, sublicensable exclusive license under relevant intellectual property rights controlled by the Company to exploit Microbiome Oncology Products for the treatment of cancer. Additionally, the Company granted to AstraZeneca an additional exclusive option to obtain a worldwide, sublicensable, license under certain intellectual property rights arising out of the Agreement or coming into the control of the Company during the term of the Agreement, to exploit AstraZeneca’s oncology and other assets which are the subject of the research plan. AstraZeneca may exercise each option at any point prior to 90 days after the end of the Exclusivity Period (the “Option Exercise Period”) by delivering an option exercise notice to the Company. If AstraZeneca exercises an option during the Option Exercise Period, the parties will enter into exclusive, good faith negotiations for a period of six months (the “Negotiation Period”) regarding the terms of the definitive license agreement contemplated by such option. If no definitive agreement is reached during the Negotiation Period, subject to certain other terms and conditions applicable for a one (1) year period, the Company is free to license, further develop or otherwise exploit its assets that were the subject of the option without further obligation to AstraZeneca.

The term of the Research Agreement continues in effect until the Research Agreement is terminated by the parties in accordance with its terms by mutual written agreement. Either party may terminate the Research Agreement for the other party’s uncured material breach or bankruptcy or insolvency-related events. AstraZeneca may terminate the Research Agreement for convenience.

Accounting Analysis

The Company assessed the Research Agreement in accordance with ASC 606 and concluded that AstraZeneca is a customer. The Company identified the following promises under the contract: (i) a research license, (ii) an obligation to perform research and development services, and (iii) participating on a joint steering committee. The Company assessed the promised goods and services to determine if they are distinct. Based on this assessment, the Company determined that AstraZeneca 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.

Each exclusive option granted to AstraZeneca provides AstraZeneca with the right to negotiate a license agreement in the future at fair value. Therefore, the Company concluded that each option does not constitute a performance obligation at inception and has been excluded from the initial allocation since each option represents a separate buying decision at market rates, rather than a material right in the contract.

At contract inception, the Company determined that the transaction price is comprised of: (i) the $20,000 fee, which represents fixed consideration, and (ii) the estimated reimbursement of research and development costs incurred, which represents variable consideration. The Company included the estimated reimbursement of research and development costs, approximately $13,900, in the transaction price at the inception of the arrangement because the Company is required to perform research and development services and the contract requires AstraZeneca to reimburse the Company for costs incurred. Also, since the related revenue would be recognized only as the costs are incurred, and the contract precludes the joint steering committee from changing the research plan without mutual agreement, the Company determined it is not probable that a significant reversal of cumulative revenue would occur.

The Company determined that revenue under the Research Agreement should be recognized over time as AstraZeneca simultaneously receives the benefit from the Company as the Company performs under the single performance obligation over time. The Company will recognize revenue for the single performance obligation using a cost-to-cost input method as the Company has concluded it best depicts the research and joint steering committee participation services performed prior to AstraZeneca’s ability to negotiate a license. Under this method, the transaction price is recognized over the contract’s entire performance period, using costs incurred relative to total estimated costs to determine the extent of progress towards completion.

For the three and six months ended June 30, 2019, the Company recognized revenue of $1,817 and $2,077, respectively, based on the measured progress under the Research Agreement. The transaction price as of June 30, 2019 was approximately $33,900.

As of June 30, 2019, there was $5,441 of deferred revenue associated with the Research Agreement, with $2,689 presented as current and $2,752 as non-current in the condensed consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next 12 months. All costs associated with the Research Agreement are recorded in research and development expense in the condensed consolidated statements of operations and comprehensive loss.

Contract Balances from Contracts with Customers

The following table presents changes in the Company’s contract assets and contract liabilities during the six months ended June 30, 2019 and 2018:

	Balance as of December 31, 2018		Additions		Deductions			Balance as of June 30, 2019
Six months ended June 30, 2019
Contract liabilities:
Deferred revenue - related party		137,259		—		(17,069	)		120,190
Deferred revenue		—		7,518		(2,077	)		5,441

	Balance as of January 1, 2018		Additions		Deductions			Balance as of June 30, 2018
Six months ended June 30, 2018
Contract liabilities:
Deferred revenue - related party		123,783		175		(8,037	)		115,921

During the three and six months ended June 30, 2019 and 2018 the Company recognized the following revenues as a result of changes in the contract liabilities balances in the respective periods (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period		11,526		4,271		17,069		8,037

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.

Leases

6 Months Ended

Jun. 30, 2019

Leases [Abstract]

Leases

The Company leases real estate, primarily laboratory, office and manufacturing space. The Company’s leases have remaining terms ranging from less than 1 year to 5 years. Certain leases include one or more options to renew, exercised at the Company’s sole discretion, with renewal terms that can extend the lease from one year to five years. The Company evaluated the renewal options in its leases to determine if it was reasonably certain that the renewal option would be exercised, and therefore should be included in the calculation of the operating lease assets and operating lease liabilities. Given the Company’s current business structure, uncertainty of future growth, and the associated impact to real estate, the Company concluded that it is not reasonably certain that any renewal options would be exercised. Therefore, the operating lease assets and lease liabilities only contemplate the initial lease terms. All of the Company’s leases qualify as operating leases. The following table summarizes the presentation in the Company’s condensed consolidated balance sheets of its operating leases:

	As of June 30, 2019
Assets:
Operating lease assets	$	12,640

Liabilities:
Operating lease liabilities	$	4,449
Operating lease liabilities, net of current portion		17,969
Total operating lease liabilities	$	22,418

The following table summarizes the effect of lease costs in the Company’s condensed consolidated statement of operations and comprehensive loss:

	For the Three Months Ended		For the Six Months Ended
	June 30, 2019		June 30, 2019
Operating lease costs	$	1,154	$	2,308
Short-term lease costs		569		1,217
Variable lease costs		860		1,586
Total lease costs	$	2,583	$	5,111

During the three and six months ended June 30, 2019, the Company recognized $1,154 and $2,308 of rental expense related to office, laboratory, and manufacturing space. During the three and six months ended June 30, 2019, the Company made cash payments of $1,648 and $3,290 for operating leases, respectively.

The minimum lease payments for the next five years and thereafter is expected to be as follows (in thousands):

	As of June 30, 2019
2019 (remaining 6 months)	$	3,304
2020		6,382
2021		6,461
2022		6,390
2023		5,158
2024 and thereafter		—
Total future minimum lease payments	$	27,695
Less: interest		(5,277	)
Present value of operating lease liabilities	$	22,418

As of June 30, 2019, the weighted average remaining lease term was 4.33 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 11%.

ASC 840 Disclosures

The future minimum lease payments under the Company’s operating leases as of December 31, 2018, were as follows (in thousands):

	As of December 31, 2018
2019	$	6,342
2020		6,120
2021		6,221
2022		6,372
2023		5,158
2024 and thereafter		—
Total future minimum lease payments	$	30,213

During the three and six months ended June 30, 2018, the Company recognized $1,107 and $2,206 of rental expense, respectively, related to office, laboratory, and manufacturing space.

Restructuring

6 Months Ended

Jun. 30, 2019

Restructuring And Related Activities [Abstract]

Restructuring

10.	Restructuring

In February 2019, the Company implemented corporate changes to focus its resources on advancing its clinical-stage therapeutic candidates. As a result, the Company now intends to 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, 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 and advancing SER-301 into clinical development. In connection with the prioritization of these therapeutic candidates, the Company made changes to its management team and reduced headcount by approximately 30 percent.

During the six months ended June 30, 2019 the Company recorded charges of $1,492, related to severance and other termination benefits. No restructuring charges were recorded during the three months ended June 30, 2019. Of that amount, the Company paid $608 and $286 during the first and second quarter of 2019, respectively, and it expects to pay approximately $598 in the remainder of 2019.

The outstanding restructuring liabilities are included in accrued expenses and other current liabilities on the condensed consolidated balance sheets. As of June 30, 2019, the components of the liabilities were as follows:

	Employee Severance and Other Benefits
Restructuring expenses	$	1,492
Cash payments	$	(894	)
Liability included in accrued expenses and other current liabilities at June 30, 2019	$	598

Income Taxes

6 Months Ended

Jun. 30, 2019

Income Tax Disclosure [Abstract]

Income Taxes

11.	Income Taxes

The Company did not provide for any income taxes for the three or six months ended June 30, 2019 and 2018.

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 June 30, 2019 and December 31, 2018. Management reevaluates the positive and negative evidence at each reporting period.

As of June 30, 2019 and December 31, 2018, the Company had no accrued interest or tax penalties recorded. The Company files income tax returns in the United States and various state jurisdictions. The Company is no longer subject to U.S. federal income tax examinations by tax authorities for years before 2012. However, to the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent it is utilized in a future period. There are no currently ongoing or pending examinations in any jurisdictions.

Commitments and Contingencies

6 Months Ended

Jun. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

12.	Commitments and Contingencies

Leases

Refer to Note 9, Leases, for discussion of the commitments associated with the Company’s lease portfolio.

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 condensed consolidated financial statements as of June 30, 2019 or December 31, 2018.

Legal Contingencies

The Company accrues a liability for legal contingencies when it believes that it is both probable that a liability has been incurred and that the Company can reasonably estimate the amount of the loss. The Company reviews these accruals and adjusts them to reflect ongoing negotiations, settlements, rulings, advice of legal counsel and other relevant information. To the extent new information is obtained and the views on the probable outcomes of claims, suits, assessments, investigations or legal proceedings change, changes in the Company’s accrued liabilities would be recorded in the period in which such determination is made.

In addition, in accordance with the relevant authoritative guidance, for any matters in which the likelihood of material loss is at least reasonably possible, the Company will provide disclosure of the possible loss or range of loss. If a reasonable estimate cannot be made, however, the Company will provide disclosure to that effect. The Company expenses legal costs as they are incurred.

The Company did not accrue any liabilities related to legal contingencies in its condensed consolidated financial statements as of June 30, 2019 or December 31, 2018.

Related Party Transactions

6 Months Ended

Jun. 30, 2019

Related Party Transactions

13.	Related Party Transactions

As described in Note 8, in January 2016 the Company entered into the License 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 three and six months ended June 30, 2019 and 2018, the Company recognized $10,454, $17,069, $4,271 and $8,037 of related party revenue associated with the License Agreement, respectively. As of June 30, 2019, there was $120,190 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 three and six months ended June 30, 2019 and 2018. There is no amount due from NHS as of June 30, 2019.

Subsequent Events

6 Months Ended

Jun. 30, 2019

Subsequent Events [Abstract]

Subsequent Events

14.	Subsequent Events

In July 2019, the Company entered into a sublease agreement with a related party to sublease a portion of its office and laboratory space in Cambridge, Massachusetts. The term of the sublease agreement commenced in July 2019 and ends on the last day of the 24^th calendar month following commencement. The annual rent for the subleased premises will be approximately $1,200 in the first year and $1,300 in the second year, which is greater than the annual rent owed by the Company to the landlord for the leased premises.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Use of Estimates

Net Loss per Share

	Three and Six Months Ended June 30,
	2019		2018
Stock options to purchase common stock		8,628,459		7,481,478
Unvested restricted stock units		130,900		289,248
Shares issuable under ESPP		37,537		—
Total common stock equivalents		8,796,896		7,770,726

Leases

Restructuring

Recently Issued Accounting Standards

	December 31, 2018		Impact of adoption of ASC 842			January 1, 2019
Operating lease assets	$	—	$	13,737		$	13,737
Accrued expenses and other current liabilities		15,207		(1,768	)		13,439
Operating lease liabilities		—		4,285			4,285
Lease incentive obligation, net of current portion		6,776		(6,776	)		—
Deferred rent		2,216		(2,216	)		—
Operating lease liabilities, net of current portion		—		20,212			20,212

•

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2019

Accounting Policies [Abstract]

Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share

	Three and Six Months Ended June 30,
	2019		2018
Stock options to purchase common stock		8,628,459		7,481,478
Unvested restricted stock units		130,900		289,248
Shares issuable under ESPP		37,537		—
Total common stock equivalents		8,796,896		7,770,726

Summary of Impact on Condensed Consolidated Balance Sheets for Opening Balances Related to Lease

The impact to the Condensed Consolidated Balance Sheets for the opening balances is as follows (in thousands):

	December 31, 2018		Impact of adoption of ASC 842			January 1, 2019
Operating lease assets	$	—	$	13,737		$	13,737
Accrued expenses and other current liabilities		15,207		(1,768	)		13,439
Operating lease liabilities		—		4,285			4,285
Lease incentive obligation, net of current portion		6,776		(6,776	)		—
Deferred rent		2,216		(2,216	)		—
Operating lease liabilities, net of current portion		—		20,212			20,212

Property and Equipment, Net (Tables)

6 Months Ended

Jun. 30, 2019

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following:

	June 30, 2019			December 31, 2018
Laboratory equipment	$	15,101		$	14,695
Computer equipment		2,874			2,864
Furniture and office equipment		1,033			1,033
Leasehold improvements		27,977			27,977
Construction in progress		26			26
		47,011			46,595
Less: Accumulated depreciation and amortization	$	(24,066	)	$	(20,301	)
	$	22,945		$	26,294

Accrued Expenses and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2019

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:

	June 30, 2019		December 31, 2018
Development and manufacturing costs	$	5,626	$	7,046
Payroll and payroll-related costs		3,516		5,020
Facility and other		1,447		3,141
	$	10,589	$	15,207

Stockholders' Deficit Common Stock (Tables)

6 Months Ended

Jun. 30, 2019

Equity [Abstract]

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity since 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, 2018		7,561,719		$	12.26	$	7.23	$	4,958
Granted		2,305,350			5.95
Exercised		(38,125	)		3.14
Forfeited		(1,200,485	)		12.08
Outstanding as of June 30, 2019		8,628,459		$	10.64	$	7.23	$	3,190
Options vested and expected to vest as of June 30, 2019		8,628,459			10.64		7.23		3,190
Options exercisable as of June 30, 2019		4,812,997		$	12.50		5.82	$	3,138

Summary of Restricted Stock Unit Activity

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock unit activity since December 31, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2018		226,900		$	9.64
Granted		—			—
Forfeited		(22,500	)		8.77
Vested		(73,500	)		9.98
Unvested restricted stock units as of June 30, 2019		130,900		$	9.60

Summary of Stock Based Compensation Expense

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development expenses	$	1,093	$	2,262	$	2,541	$	4,190
General and administrative expenses		1,004		2,072		1,621		4,380
	$	2,097	$	4,334	$	4,162	$	8,570

Collaboration Revenue (Tables)

6 Months Ended

Jun. 30, 2019

Revenue From Contract With Customer [Abstract]

Changes in Contract Assets and Contract Liabilities

The following table presents changes in the Company’s contract assets and contract liabilities during the six months ended June 30, 2019 and 2018:

	Balance as of December 31, 2018		Additions		Deductions			Balance as of June 30, 2019
Six months ended June 30, 2019
Contract liabilities:
Deferred revenue - related party		137,259		—		(17,069	)		120,190
Deferred revenue		—		7,518		(2,077	)		5,441

	Balance as of January 1, 2018		Additions		Deductions			Balance as of June 30, 2018
Six months ended June 30, 2018
Contract liabilities:
Deferred revenue - related party		123,783		175		(8,037	)		115,921

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period		11,526		4,271		17,069		8,037

Leases (Tables)

6 Months Ended

Jun. 30, 2019

Leases [Abstract]

Summary of Operating Lease Assets and Liabilities

The following table summarizes the presentation in the Company’s condensed consolidated balance sheets of its operating leases:

	As of June 30, 2019
Assets:
Operating lease assets	$	12,640

Liabilities:
Operating lease liabilities	$	4,449
Operating lease liabilities, net of current portion		17,969
Total operating lease liabilities	$	22,418

Summary of Lease Costs

The following table summarizes the effect of lease costs in the Company’s condensed consolidated statement of operations and comprehensive loss:

	For the Three Months Ended		For the Six Months Ended
	June 30, 2019		June 30, 2019
Operating lease costs	$	1,154	$	2,308
Short-term lease costs		569		1,217
Variable lease costs		860		1,586
Total lease costs	$	2,583	$	5,111

Schedule of Minimum Lease Payments

The minimum lease payments for the next five years and thereafter is expected to be as follows (in thousands):

	As of June 30, 2019
2019 (remaining 6 months)	$	3,304
2020		6,382
2021		6,461
2022		6,390
2023		5,158
2024 and thereafter		—
Total future minimum lease payments	$	27,695
Less: interest		(5,277	)
Present value of operating lease liabilities	$	22,418

Summary of Future Minimum Lease Payments Under Operating Leases

The future minimum lease payments under the Company’s operating leases as of December 31, 2018, were as follows (in thousands):

	As of December 31, 2018
2019	$	6,342
2020		6,120
2021		6,221
2022		6,372
2023		5,158
2024 and thereafter		—
Total future minimum lease payments	$	30,213

Restructuring (Tables)

6 Months Ended

Jun. 30, 2019

Restructuring And Related Activities [Abstract]

Schedule of Components of Liabilities

As of June 30, 2019, the components of the liabilities were as follows:

	Employee Severance and Other Benefits
Restructuring expenses	$	1,492
Cash payments	$	(894	)
Liability included in accrued expenses and other current liabilities at June 30, 2019	$	598

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended				6 Months Ended
	Jun. 21, 2019	Jun. 18, 2019	Feb. 28, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporated year								Oct. 31, 2010
Entity incorporated state name								State of Delaware
Percent of reduction in headcount of employees			30.00%
Aggregate net proceeds from offering of common stock								$ 60,759
Accumulated deficit				$ 424,462				424,462		$ 389,370
Net loss				(10,759)	$ (24,333)	$ (27,787)	$ (27,919)	(35,092)	$ (55,706)
Cash used in operations								(44,181)	$ (52,437)
Cash and cash equivalents				$ 102,230				$ 102,230		$ 85,820
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued				28,818,578
Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Aggregate net proceeds from offering of common stock	$ 4,551	$ 55,976
Underwritten Public Offering [Member] \| Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, shares issued	2,151,911	26,666,667
Common stock, share price	$ 2.25	$ 2.25
Underwritten Public Offering [Member] \| Common Stock [Member] \| Maximum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Option to purchase additional shares of common stock		2,666,666

Summary of Significant Accounting Policies - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	8,796,896	7,770,726	8,796,896	7,770,726
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	8,628,459	7,481,478	8,628,459	7,481,478
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	130,900	289,248	130,900	289,248
Shares Issuable under ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents	37,537		37,537

Summary of Significant Accounting Policies -Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jan. 01, 2019
Leases [Line Items]
Lease, practical expedient	false
Operating lease assets	$ 12,640
Operating lease liability	$ 22,418
As reported under Topic 842 [Member]
Leases [Line Items]
Operating lease assets		$ 13,737
Operating lease liability		$ 24,497

Summary of Significant Accounting Policies - Summary of Impact on Condensed Consolidated Balance Sheets for Opening Balances Related to Lease (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Leases [Line Items]
Operating lease assets	$ 12,640
Accrued expenses and other current liabilities	10,589		$ 15,207
Operating lease liabilities	4,449
Lease incentive obligation, net of current portion			6,776
Deferred rent			$ 2,216
Operating lease liabilities, net of current portion	$ 17,969
As reported under Topic 842 [Member]
Leases [Line Items]
Operating lease assets		$ 13,737
Accrued expenses and other current liabilities		13,439
Operating lease liabilities		4,285
Operating lease liabilities, net of current portion		20,212
Impact of adoption of ASC 842 [Member] \| As reported under Topic 842 [Member]
Leases [Line Items]
Operating lease assets		13,737
Accrued expenses and other current liabilities		(1,768)
Operating lease liabilities		4,285
Lease incentive obligation, net of current portion		(6,776)
Deferred rent		(2,216)
Operating lease liabilities, net of current portion		$ 20,212

Fair Value Measurements - Additional Information (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted investments	$ 1,400,000	$ 1,400,000
Fair value of liabilities on recurring basis	0	0
Level 1 [Member] \| Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	20,393,000	39,982,000
Level 2 [Member] \| Certificate of Deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted investments	$ 1,400,000	$ 1,400,000

Investments - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments [Abstract]
Restricted investments	$ 1,400	$ 1,400

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 47,011	$ 46,595
Less: Accumulated depreciation and amortization	(24,066)	(20,301)
Property and equipment, net	22,945	26,294
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	15,101	14,695
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,874	2,864
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	27,977
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 26	$ 26

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,921	$ 1,954	$ 3,927	$ 3,895

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Development and manufacturing costs	$ 5,626	$ 7,046
Payroll and payroll-related costs	3,516	5,020
Facility and other	1,447	3,141
Total accrued expenses and other current liabilities	$ 10,589	$ 15,207

Stockholders' Deficit Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Equity [Abstract]
Number of Shares, Beginning Balance	7,561,719
Number of Shares, Granted	2,305,350
Number of Shares, Exercised	(38,125)
Number of Shares, Forfeited	(1,200,485)
Number of Shares, Ending Balance	8,628,459	7,561,719
Number of Shares, Options vested and expected to vest	8,628,459
Number of Shares, Options exercisable	4,812,997
Weighted Average Exercise Price, Beginning Balance	$ 12.26
Weighted Average Exercise Price, Granted	5.95
Weighted Average Exercise Price, Exercised	3.14
Weighted Average Exercise Price, Forfeited	12.08
Weighted Average Exercise Price, Ending Balance	10.64	$ 12.26
Weighted Average Exercise Price, Options vested and expected to vest	10.64
Weighted Average Exercise Price, Options exercisable	$ 12.50
Weighted Average Remaining Contractual Term, Outstanding	7 years 2 months 23 days	7 years 2 months 23 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	5 years 9 months 25 days
Aggregate Intrinsic Value, Outstanding	$ 3,190	$ 4,958
Aggregate Intrinsic Value, Options vested and expected to vest	3,190
Aggregate Intrinsic Value, Options exercisable	$ 3,138

Stockholders' Deficit Common Stock - Stock Options - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options	$ 2.38	$ 6.89	$ 4.34	$ 5.82
Performance-based stock options to granted			2,305,350
Stock based compensation expense for stock options	$ 2,097,000	$ 4,334,000	$ 4,162,000	$ 8,570,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value of stock options			$ 4.58
Performance-based stock options to granted			1,100,000
Stock options exercisable	0		0
Stock based compensation expense for stock options			$ 0

Stockholders' Deficit Common Stock - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance \| shares	226,900
Number of Shares, Forfeited \| shares	(22,500)
Number of Shares, Vested \| shares	(73,500)
Number of Shares, Unvested restricted stock units, Ending balance \| shares	130,900
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance \| $ / shares	$ 9.64
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	8.77
Weighted Average Grant Date Fair Value, Vested \| $ / shares	9.98
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance \| $ / shares	$ 9.60

Stockholders' Deficit Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,097	$ 4,334	$ 4,162	$ 8,570
Research and development expenses [Member] \| Stock Options, Restricted Stock Units and Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,093	2,262	2,541	4,190
General and administrative expenses [Member] \| Stock Options, Restricted Stock Units and Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 1,004	$ 2,072	$ 1,621	$ 4,380

Stockholders' Deficit Common Stock - Employee Stock Purchase Plan - Additional Information (Detail) - Employee Stock Purchase Plan [Member]	6 Months Ended
	Jun. 30, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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%
Aggregate shares of common stock reserved for issuance	1,900,000
Shares issued under ESPP	400,000
Percentage of common stock shares outstanding	1.00%

Collaboration Revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
Collaboration Revenue - Additional Information (Detail)	Mar. 31, 2019 USD ($) Installment	Dec. 31, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2018 USD ($)	Feb. 29, 2016 USD ($)	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party			$ 10,454,000	$ 4,271,000	$ 17,069,000	$ 8,037,000
Deferred revenue, current			2,689,000		2,689,000
Deferred revenue, non-current			2,752,000		2,752,000
Research Agreement [Member] \| AstraZeneca Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment			5,441,000		5,441,000
Upfront collaboration milestone payments receivable	$ 20,000,000
Transaction price allocated to remaining performance obligations			33,900,000		33,900,000
Collaboration revenue - related party			1,817,000		$ 2,077,000
Minimum exclusivity period	3 years
Number of installments \| Installment	3
Option exercise period from exclusivity period					90 days
Terms of the definitive license agreement good faith negotiation period					6 months
Other terms of definitive license agreement period					1 year
Research and development fixed consideration			20,000,000		$ 20,000,000
Research and development estimated reimbursement costs					13,900,000
Deferred revenue, current			2,689,000		2,689,000
Deferred revenue, non-current			2,752,000		2,752,000
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment			120,190,000		120,190,000						$ 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										$ 40,000,000
Proceeds on achievement of development milestone		$ 40,000,000					$ 40,000,000	$ 20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations			190,000,000		190,000,000
Collaboration revenue - related party			10,454,000	$ 4,271,000	17,069,000	$ 8,037,000
Deferred revenue		$ 137,259,000	120,190,000		120,190,000		$ 137,259,000
Nestlé Health Science [Member] \| Phase 2 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable												$ 20,000,000
Nestlé Health Science [Member] \| Phase 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable												$ 20,000,000
Collaboration revenue - related party			$ 6,830,000		$ 6,830,000

Collaboration Revenue - Changes in Contract Assets and Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party, Deductions	$ (11,526)	$ (4,271)	$ (17,069)	$ (8,037)
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party, Balance at beginning of period			137,259	123,783
Deferred revenue - related party, Additions				175
Deferred revenue - related party, Deductions			(17,069)	(8,037)
Deferred revenue - related party, Balance at end of period	120,190	$ 115,921	120,190	$ 115,921
Deferred revenue, Additions			7,518
Deferred revenue, Deductions			(2,077)
Deferred revenue, Balance at end of period	$ 5,441		$ 5,441

Collaboration Revenue - Schedule of Revenue Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 11,526	$ 4,271	$ 17,069	$ 8,037

Leases - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating Leased Assets [Line Items]
Existence of option to extend			true
Option to extend, description			Certain leases include one or more options to renew, exercised at the Company’s sole discretion, with renewal terms that can extend the lease from one year to five years.
Cash payments for operating leases	$ 1,648		$ 3,290
Weighted average remaining lease term	4 years 3 months 29 days		4 years 3 months 29 days
Weighted average incremental borrowing rate	11.00%		11.00%
Office, Laboratory and Manufacturing Space [Member]
Operating Leased Assets [Line Items]
Rental expense related to office and laboratory space	$ 1,154	$ 1,107	$ 2,308	$ 2,206
Minimum [Member]
Operating Leased Assets [Line Items]
Operating lease, remaining term			1 year
Lease extension period	1 year		1 year
Maximum [Member]
Operating Leased Assets [Line Items]
Operating lease, remaining term			5 years
Lease extension period	5 years		5 years

Leases - Summary of Operating Lease Assets and Liabilities (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Assets
Operating lease assets	$ 12,640
Liabilities:
Operating lease liabilities	4,449
Operating lease liabilities, net of current portion	17,969
Total operating lease liabilities	$ 22,418

Leases - Summary of Lease Costs (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Leases [Abstract]
Operating lease costs	$ 1,154	$ 2,308
Short-term lease costs	569	1,217
Variable lease costs	860	1,586
Total lease costs	$ 2,583	$ 5,111

Leases - Schedule of Minimum Lease Payments (Detail) $ in Thousands	Jun. 30, 2019 USD ($)
Leases [Abstract]
2019 (remaining 6 months)	$ 3,304
2020	6,382
2021	6,461
2022	6,390
2023	5,158
Total future minimum lease payments	27,695
Less: interest	(5,277)
Total operating lease liabilities	$ 22,418

Leases - Summary of Future Minimum Lease Payments Under Operating Leases (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 6,342
2020	6,120
2021	6,221
2022	6,372
2023	5,158
Total future minimum lease payments	$ 30,213

Restructuring - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019	Dec. 31, 2019
Restructuring Cost And Reserve [Line Items]
Percent of reduction in headcount of employees	30.00%
Restructuring Charges		$ 0		$ 1,492,000
Cash payments		$ 286,000	$ 608,000
Scenario Forecast [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring liabilities expected to pay					$ 598,000
Severance And Other Termination Benefits [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring Charges				$ 1,492,000

Restructuring - Schedule of Components of Liabilities (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019
Restructuring Cost And Reserve [Line Items]
Restructuring expenses	$ 0		$ 1,492,000
Cash payments	(286,000)	$ (608,000)
Employee Severance And Other Benefits [Member] \| Accrued Expenses and Other Current Liabilities [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses			1,492,000
Cash payments			(894,000)
Liability included in accrued expenses and other current liabilities at June 30, 2019	$ 598,000		$ 598,000

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 0	$ 0	$ 0	$ 0
Accrued interest or tax penalties	$ 0		$ 0		$ 0

Commitments and Contingencies - Additional Information (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Indemnification Agreement [Member]
Other Commitments [Line Items]
Indemnification obligations accrued	$ 0	$ 0

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 10,454,000	$ 4,271,000	$ 17,069,000	$ 8,037,000
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Collaboration revenue - related party	10,454,000	4,271,000	17,069,000	8,037,000
Deferred revenue	120,190,000		120,190,000		$ 120,000,000
Payments under agreements with related party	0	$ 0		$ 0
Due from related party for the reimbursement of development costs	$ 0		$ 0

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] $ in Thousands	Jul. 31, 2019 USD ($)
Subsequent Event [Line Items]
Sublease annual rent first year	$ 1,200
Sublease annual rent second year	$ 1,300