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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2017	Jul. 31, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q2
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	0001609809
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		40,512,639

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 33,642	$ 54,539
Investments	137,570	138,704
Prepaid expenses and other current assets	4,839	5,126
Total current assets	176,051	198,369
Property and equipment, net	34,813	36,125
Long-term investments	3,962	36,752
Restricted cash	1,513	1,400
Total assets	216,339	272,646
Current liabilities:
Accounts payable	3,889	7,587
Accrued expenses and other current liabilities	9,718	10,812
Deferred revenue - related party	12,058	12,058
Total current liabilities	25,665	30,457
Lease incentive obligation, net of current portion	9,859	10,730
Deferred rent	2,159	2,072
Deferred revenue, net of current portion - related party	90,727	96,756
Total liabilities	128,410	140,015
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at June 30, 2017 and December 31, 2016; no shares issued and outstanding at June 30, 2017 and December 31, 2016
Common stock, $0.001 par value; 200,000,000 shares authorized at June 30, 2017 and December 31, 2016; 40,412,035 and 40,355,753 shares issued and outstanding at June 30, 2017 and December 31, 2016, respectively	40	40
Additional paid-in capital	315,748	306,931
Accumulated other comprehensive loss	(176)	(149)
Accumulated deficit	(227,683)	(174,191)
Total stockholders’ equity	87,929	132,631
Total liabilities and stockholders’ equity	$ 216,339	$ 272,646

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	40,412,035	40,355,753
Common stock, shares outstanding	40,412,035	40,355,753

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue:
Collaboration revenue - related party	$ 3,014	$ 3,004	$ 6,029	$ 5,714
Total revenue	3,014	3,004	6,029	5,714
Operating expenses:
Research and development expenses	23,060	22,174	43,203	37,590
General and administrative expenses	8,370	8,970	17,132	16,180
Total operating expenses	31,430	31,144	60,335	53,770
Loss from operations	(28,416)	(28,140)	(54,306)	(48,056)
Other income (expense):
Interest income	615	495	1,390	763
Other income (expense)	(217)	(268)	(576)	(324)
Total other income, net	398	227	814	439
Net loss	$ (28,018)	$ (27,913)	$ (53,492)	$ (47,617)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.69)	$ (0.70)	$ (1.32)	$ (1.21)
Weighted average common shares outstanding, basic and diluted	40,394,605	39,600,344	40,381,643	39,393,238
Other comprehensive (loss) income:
Unrealized (loss) gain on investments, net of tax of $0	$ (25)	$ (25)	$ (27)	$ 53
Total other comprehensive (loss) income	(25)	(25)	(27)	53
Comprehensive loss	$ (28,043)	$ (27,938)	$ (53,519)	$ (47,564)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (53,492)	$ (47,617)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,781	8,621
Depreciation and amortization expense	3,490	1,260
Non-cash interest expense	2	1
Accretion of discount on investments	(116)	(222)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	287	(2,719)
Deferred revenue	(6,029)	114,383
Accounts payable	(3,651)	395
Accrued expenses and other current liabilities	(577)	1,991
Net cash provided by (used in) operating activities	(51,305)	76,093
Cash flows from investing activities:
Purchases of property and equipment	(3,527)	(9,946)
Purchases of investments	(43,795)	(206,534)
Sales and maturities of investments	77,807	125,335
Changes in restricted cash	(113)	(1)
Net cash provided by (used in) investing activities	30,372	(91,146)
Cash flows from financing activities:
Proceeds from exercise of stock options	36	944
Net cash provided by financing activities	36	944
Net decrease in cash and cash equivalents	(20,897)	(14,109)
Cash and cash equivalents at beginning of period	54,539	73,933
Cash and cash equivalents at end of period	33,642	59,824
Supplemental disclosure of cash flow information:
Cash paid for interest		108
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 140	$ 4,899

Nature of the Business and Basis of Presentation

6 Months Ended

Jun. 30, 2017

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 restore health by repairing the function of a dysbiotic microbiome. The Company’s lead product candidate, SER-109, is designed to prevent further recurrences of Clostridium difficile infection (“CDI”), a debilitating infection of the colon, and, if approved by the U.S. Food and Drug Administration (“FDA”), could be a first-in-field oral microbiome drug. Using its microbiome therapeutics platform, the Company is developing additional product candidates to treat diseases where the microbiome is implicated, including SER-262, a synthetic product candidate, to prevent an initial recurrence of primary CDI, SER-287 to treat ulcerative colitis, SER-301, a synthetic inflammatory bowel disease product candidate, and SER-155, a synthetic product candidate to prevent infections and improve gastrointestinal barrier function (including the consequences of graft versus host disease) in patients following allogeneic hematopoietic stem cell transplants or solid organ transplants. The Company is also using its microbiome therapeutics platform to conduct research on metabolic diseases, such as non-alcoholic steatohepatitis (NASH); inflammatory diseases, such as Crohn’s disease; rare liver disorders such as primary sclerosing cholangitis (PSC); and immuno-oncology treatments.

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, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants.

The Company’s consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. The Company has experienced negative cash flows and had an accumulated deficit of $227,683 and $174,191 as of June 30, 2017 and December 31, 2016, respectively. The Company expects that its cash, cash equivalents and investments at June 30, 2017 of $175,174 will enable it to fund its operating expense and capital expenditure requirements through 2018. The future viability of the Company is dependent on its ability to generate cash from operating activities or to raise additional capital to finance its operations. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies.

Unaudited Interim Financial Information

The accompanying unaudited consolidated financial statements as of June 30, 2017 and for the three and six months ended June 30, 2017 and 2016 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 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, 2016 included in the Company’s annual report on Form 10-K for the fiscal year ended December 31, 2016, which was filed with the SEC on March 16, 2017.

The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. The condensed consolidated balance sheet at December 31, 2016 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 statement of the Company’s financial position as of June 30, 2017 and consolidated results of operations for the three and six months ended June 30, 2017 and its cash flows for the six months ended June 30, 2017 and 2016. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2017 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2017.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2017

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, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2017.

During the three and six months ended June 30, 2017, the Company recorded revenue in connection with its collaboration agreement. See Note 9, “Collaboration Revenue,” for additional information.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

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 warrants and unvested restricted stock.

The restricted stock units granted by the Company entitle the holder of such awards to dividends declared or paid by the board of directors, regardless of whether such awards are unvested, as if such shares were outstanding common shares at the time of the dividend. However, the unvested restricted stock awards are not entitled to share in the residual net assets (deficit) of the Company. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive.

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,
	2017		2016
Stock options to purchase common stock		6,314,838		5,612,389
Unvested restricted stock units		408,400		—
		6,723,238		5,612,389

Recently Issued Accounting Standards

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which delayed the effective date of the new standard from January 1, 2017 to January 1, 2018. The FASB also agreed to allow entities to choose to adopt the standard as of the original effective date. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations, which clarifies the implementation guidance on principal versus agent considerations. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies how a company identifies promised goods or services and clarifies whether an entity’s promise to grant a license provides a customer with either a right to use the entity’s intellectual property (which is satisfied at a point in time) or a right to access the entity’s intellectual property (which is satisfied over time). In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients related to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. In December 2016 the FASB issued ASU No. 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, which amends certain narrow aspects of the guidance issued in ASU 2014-09 including guidance related to the disclosure of remaining performance obligations and prior-period performance obligations, as well as other amendments to the guidance on loan guarantee fees, contract costs, refund liabilities, advertising costs and the clarification of certain examples. We are currently evaluating the method of adoption and the potential impact that Topic 606 may have on the Company’s financial position and results of operation.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASC 842 supersedes the previous leases standard, ASC 840 Leases. The standard is effective on January 1, 2019, with early adoption permitted. The Company is in the process of evaluating the impact of this new guidance.

In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments. The new standard addresses specific cash flow issues with the objective of reducing existing diversity in practice. The new standard will be effective for the Company on January 1, 2018. The Company is in the process of evaluating the impact of this new guidance.

In November 2016, the FASB issued ASU 2016-18, Restricted Cash. The new standard requires restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the total beginning and ending amounts for the periods shown on the statement of cash flows. The new standard will be effective for the Company on January 1, 2018. The Company is in the process of evaluating the impact of this new guidance.

In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting. The new standard provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The new standard will be effective for the Company on January 1, 2018. The Company is in the process of evaluating the impact of this new guidance.

Fair Value Measurements

6 Months Ended

Jun. 30, 2017

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.

The Company’s cash equivalents and investments are carried at fair value, determined according to the fair value hierarchy described above. The Company’s investments in certificates of deposit are carried at amortized cost, which approximates fair value. Certain cash equivalents or investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The carrying values of the Company’s accounts payable and accrued expenses approximate their fair value due to the short-term nature of these liabilities.

The following table presents information about the Company’s assets as of June 30, 2017 and December 31, 2016 that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (note there were no liabilities measured at fair value on a recurring basis in either of the periods presented):

	Fair Value Measurements as of June 30, 2017 Using:
	Level 1		Level 2		Level 3		Not Subject to Leveling ⁽¹⁾		Total
Assets:
Cash Equivalents	$	—	$	1,000	$	—	$	2,317	$	3,317
Investments:
Commercial Paper	$	—	$	16,699	$	—	$	—	$	16,699
Certificates of Deposit		—		8,174		—		—		8,174
Corporate Bonds		—		74,174		—		—		74,174
Government Securities		—		26,431		—		—		26,431
Treasury Bonds		—		16,054		—		—		16,054
	$	—	$	142,532	$	—	$	2,317	$	144,849
⁽¹⁾ Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

	Fair Value Measurements as of December 31, 2016 Using:
	Level 1		Level 2		Level 3		Not Subject to Leveling ⁽¹⁾		Total
Assets:
Cash Equivalents	$	—	$	4,740	$	—	$	1,567	$	6,307
Repurchase Agreements		—		7,000		—		—		7,000
Investments:
Commercial Paper	$	—	$	19,689	$	—	$	—	$	19,689
Certificates of Deposit		—		10,629						10,629
Corporate Bonds		—		94,609		—		—		94,609
Government Securities		—		33,466		—		—		33,466
Treasury Bonds		—		17,063		—		—		17,063
	$	—	$	187,196	$	—	$	1,567	$	188,763
⁽¹⁾ Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

As of June 30, 2017, the Company’s cash equivalents, which were invested in money market funds and corporate bonds with original maturities of less than 90 days from the date of purchase, were valued based on Level 2 inputs.

As of December 31, 2016, the Company’s cash equivalents consisted of money market funds, corporate bonds, certificates of deposit, and repurchase agreements with original maturities of less than 90 days from the date of purchase and were valued based on Level 2 inputs. Repurchase agreements are agreements with banks to repurchase notes that are collateralized by U.S. government securities. All repurchase agreements have overnight maturities.

The fair value of the Company’s investments, which consisted of commercial paper, certificates of deposit, corporate bonds, government securities and treasury bonds as of June 30, 2017 and December 31, 2016 were determined using Level 2 inputs. During the three and six months ended June 30, 2017 and 2016 there were no transfers between Level 1, Level 2 and Level 3.

- 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, 2017

Investments Debt And Equity Securities [Abstract]

Investments

4.	Investments

As of June 30, 2017 and December 31, 2016, the fair value of available-for-sale investments by type of security was as follows:

	June 30, 2017
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial Paper	$	16,705	$	—	$	(6	)	$	16,699
Certificates of Deposit		8,174		—		—			8,174
Corporate Bonds		74,261		—		(87	)		74,174
Government Securities		26,500		—		(69	)		26,431
Treasury Bonds		16,067		—		(13	)		16,054
	$	141,707	$	—	$	(175	)	$	141,532

	December 31, 2016
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial Paper	$	19,631	$	58	$	—		$	19,689
Certificates of Deposit		10,629		—		—			10,629
Corporate Bonds		94,764		—		(155	)		94,609
Government Securities		33,513		—		(47	)		33,466
Treasury Bonds		17,066		1		(4	)		17,063
	$	175,603	$	59	$	(206	)	$	175,456

Investments with original maturities of less than 90 days are included in cash and cash equivalents on the consolidated balance sheets and are not included in the table above. Investments with maturities of less than 12 months are considered current and those investments with maturities greater than 12 months are considered non-current.

Property and Equipment, Net

6 Months Ended

Jun. 30, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

5.	Property and Equipment, Net

Property and equipment, net consisted of the following:

	June 30, 2017			December 31, 2016
Laboratory equipment	$	11,802		$	10,711
Computer equipment		1,366			1,335
Furniture and office equipment		1,033			1,010
Leasehold improvements		27,896			27,807
Construction in progress		1,387			442
		43,484			41,305
Less: Accumulated depreciation and amortization		(8,671	)		(5,180	)
	$	34,813		$	36,125

Construction in progress at June 30, 2017 was comprised primarily of computer equipment related to data storage and security.

Depreciation and amortization expense was $1,760, $3,490, $874, and $1,260 for the three and six months ended June 30, 2017 and 2016, respectively.

Stockholders' Equity Common Stock

6 Months Ended

Jun. 30, 2017

Equity [Abstract]

Stockholders' Equity Common Stock

8.	Stockholders’ Equity Common Stock

Stock Options

The following table summarizes the Company’s stock option activity since December 31, 2016:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2016		5,069,133		$	14.36		8.26	$	16,736
Granted		1,609,500			10.07
Exercised		(56,282	)		0.65
Forfeited		(307,513	)		20.18
Outstanding as of June 30, 2017		6,314,838		$	13.11		8.15	$	20,875
Options exercisable as of June 30, 2017		2,800,162		$	11.10		7.34	$	14,634

The weighted average grant-date fair value of stock options granted during the three and six months ended June 30, 2017 and 2016 was $7.44, $6.98, $20.64, and $19.87 per share, respectively.

Restricted Stock Units

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the six months ended June 30, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2016		115,500		$	9.78
Granted		332,500		$	10.08
Forfeited		(39,600	)	$	9.96
Vested		—		$	—
Unvested restricted stock units as of June 30, 2017		408,400		$	10.01

Stock-based Compensation Expense

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its condensed consolidated statements of operations and comprehensive loss:

	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development expenses	$	2,094	$	2,896	$	3,967	$	5,082
General and administrative expenses		2,409		1,821		4,814		3,539
	$	4,503	$	4,717	$	8,781	$	8,621

Collaboration Revenue

6 Months Ended

Jun. 30, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaboration Revenue

9.	Collaboration Revenue

Nestec Ltd.

In January 2016 the Company entered into the License Agreement with NHS, an affiliate of Nestlé Health Science US Holdings, Inc., a significant stockholder of the Company, for the development and commercialization of certain product candidates in development for the treatment and management of CDI and inflammatory bowel disease (“IBD”), including ulcerative colitis and Crohn’s disease. The License Agreement will support 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.

In exchange for the license, NHS agreed to pay the Company an upfront cash payment of $120,000, which the Company received in February 2016. 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. 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.

At the inception of the License Agreement, the Company identified the following deliverables: (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. The Company also identified a contingent deliverable, the obligation to perform manufacturing services to provide commercial supply if commercialization occurs, which is contingent upon regulatory approval. This contingent deliverable has been excluded from the initial allocation and will be treated as a separate unit of accounting when and if delivered.

The Company concluded that none of the four deliverables identified at the inception of the License Agreement has standalone value from the other undelivered elements. Accordingly, all deliverables represent a single unit of accounting.

All consideration received relating to the four identified deliverables that comprise the single unit of accounting will be recognized over the period of performance. The period of performance will be through the completion of development services for the NHS Collaboration Products which has been estimated to be ten years. The Company will periodically review and, if necessary, revise the estimated development period.

The Company will recognize revenue utilizing a time-based proportional performance model where revenue related to each payment is recognized over the ten-year performance period. As of June 30, 2017, the only consideration that is fixed and determinable is the non-refundable upfront payment of $120,000 and $580 for the reimbursement of development services since the inception of the arrangement. For additional consideration that could be received for research and development services and/or manufacturing services for clinical supply, the Company will recognize a cumulative catch-up for the amount of time that has elapsed and spread the unrecognized portion over the remaining performance period.

Development and regulatory milestones that involve substantial effort on the Company’s part and the achievement of which are not considered probable at the inception of the License Agreement are considered substantive milestones, and will be recognized in their entirety in the period in which the milestone is achieved, assuming all other revenue recognition criteria are met. All commercial milestones will be recorded as revenue upon achievement of the milestone, assuming all other revenue recognition criteria are met.

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. The Company recognizes revenue associated with substantive milestones in accordance with FASB ASC Topic 605-28, Revenue Recognition-Milestone Method. The $10,000 was recognized in full as related party collaboration revenue during the year ended December 31, 2016.

Royalties will be recorded as revenue in the period they are earned assuming all other revenue recognition criteria are met.

During the three and six months ended June 30, 2017 and 2016, the Company recognized $3,014, $6,029, $3,004, and $5,714, respectively, of related party revenue (see Note 12 “Related Party Transactions”) associated with the License Agreement. As of June 30, 2017, there was $102,785 of deferred revenue related to the License Agreement, which is classified as current or non-current in the consolidated balance sheets based on the Company’s estimate of revenue that will be recognized within the next twelve months. All costs associated with the License Agreement are recorded in research and development expense in the condensed consolidated statements of operations and comprehensive loss.

Commitments and Contingencies

6 Months Ended

Jun. 30, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11.	Commitments and Contingencies

Leases

On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016. BMR has contributed a total of $12,509 toward the cost of tenant improvements. BMR’s contributions toward the cost of tenant improvements is recorded as a lease incentive obligation on the Company’s consolidated balance sheet. The lease incentive obligation is amortized to the Company’s consolidated statement of operations as reductions to rent expense over the lease term. As of June 30, 2017, the Company has recorded a lease incentive obligation of $11,196. During the six months ended June 30, 2017, we amortized $884 of this lease incentive obligation as a reduction to rent expense.

During the three and six months ended June 30, 2017 and 2016, the Company recognized $1,145, $2,244, $787, and $1,180 respectively, of rental expense related to office and laboratory space.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of June 30, 2017 or December 31, 2016.

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.

On September 28, 2016, a purported stockholder of the Company filed a putative class action lawsuit in the U.S. District Court for the District of Massachusetts against the Company entitled Mariusz Mazurek v. Seres Therapeutics, Inc., et.al. On February 12, 2017, the Company received an amended complaint and on March 30, 2017, the Company filed a motion to dismiss. A hearing on the motion to dismiss is scheduled for August 9, 2017. The lawsuit alleges violations of Sections 10(b), 20(a) and Rule 10b-5 of the Securities Exchange Act of 1934, as amended, by making allegedly false and misleading statements and omissions about the Company’s clinical trials for its product candidate SER-109 in the Company’s public disclosures between June 25, 2015 and July 29, 2016. The lawsuit seeks, among other things, damages in connection with the Company’s allegedly inflated stock price between June 25, 2015 and July 29, 2016 as a result of those allegedly false and misleading statements, as well as interest, attorneys’ fees and costs. The Company can make no assurances as to the time or resources that will need to be devoted to this lawsuit or its final outcome, or the impact, if any, of this lawsuit or any proceedings on its business, financial condition, results of operations and cash flows. While the Company is vigorously defending against all claims asserted, this litigation could result in substantial costs to the Company and a diversion of the Company’s management’s attention and resources, which could harm its business. In addition, the uncertainty of the pending lawsuit or potential filing of additional lawsuits could lead to more volatility and a reduction in the Company’s stock price. Given the early stage of the litigation, at this time the Company is unable to reasonably estimate possible losses or form a judgment that an unfavorable outcome is either probable or remote. It is not currently possible to assess whether or not the outcome of these proceedings may have a material adverse effect on the Company.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2017

Accounting Policies [Abstract]

Use of Estimates

Net Loss per Share

	Three and Six Months Ended June 30,
	2017		2016
Stock options to purchase common stock		6,314,838		5,612,389
Unvested restricted stock units		408,400		—
		6,723,238		5,612,389

Recently Issued Accounting Standards

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2017

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis

	Fair Value Measurements as of June 30, 2017 Using:
	Level 1		Level 2		Level 3		Not Subject to Leveling ⁽¹⁾		Total
Assets:
Cash Equivalents	$	—	$	1,000	$	—	$	2,317	$	3,317
Investments:
Commercial Paper	$	—	$	16,699	$	—	$	—	$	16,699
Certificates of Deposit		—		8,174		—		—		8,174
Corporate Bonds		—		74,174		—		—		74,174
Government Securities		—		26,431		—		—		26,431
Treasury Bonds		—		16,054		—		—		16,054
	$	—	$	142,532	$	—	$	2,317	$	144,849
⁽¹⁾ Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

	Fair Value Measurements as of December 31, 2016 Using:
	Level 1		Level 2		Level 3		Not Subject to Leveling ⁽¹⁾		Total
Assets:
Cash Equivalents	$	—	$	4,740	$	—	$	1,567	$	6,307
Repurchase Agreements		—		7,000		—		—		7,000
Investments:
Commercial Paper	$	—	$	19,689	$	—	$	—	$	19,689
Certificates of Deposit		—		10,629						10,629
Corporate Bonds		—		94,609		—		—		94,609
Government Securities		—		33,466		—		—		33,466
Treasury Bonds		—		17,063		—		—		17,063
	$	—	$	187,196	$	—	$	1,567	$	188,763
⁽¹⁾ Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

Investments (Tables)

6 Months Ended

Jun. 30, 2017

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value of Available-for-Sale Investments by Type of Security

As of June 30, 2017 and December 31, 2016, the fair value of available-for-sale investments by type of security was as follows:

	June 30, 2017
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial Paper	$	16,705	$	—	$	(6	)	$	16,699
Certificates of Deposit		8,174		—		—			8,174
Corporate Bonds		74,261		—		(87	)		74,174
Government Securities		26,500		—		(69	)		26,431
Treasury Bonds		16,067		—		(13	)		16,054
	$	141,707	$	—	$	(175	)	$	141,532

	December 31, 2016
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Commercial Paper	$	19,631	$	58	$	—		$	19,689
Certificates of Deposit		10,629		—		—			10,629
Corporate Bonds		94,764		—		(155	)		94,609
Government Securities		33,513		—		(47	)		33,466
Treasury Bonds		17,066		1		(4	)		17,063
	$	175,603	$	59	$	(206	)	$	175,456

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Stockholders' Equity Common Stock (Tables)

6 Months Ended

Jun. 30, 2017

Equity [Abstract]

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity since December 31, 2016:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2016		5,069,133		$	14.36		8.26	$	16,736
Granted		1,609,500			10.07
Exercised		(56,282	)		0.65
Forfeited		(307,513	)		20.18
Outstanding as of June 30, 2017		6,314,838		$	13.11		8.15	$	20,875
Options exercisable as of June 30, 2017		2,800,162		$	11.10		7.34	$	14,634

Summary of Restricted Stock Activity

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the six months ended June 30, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2016		115,500		$	9.78
Granted		332,500		$	10.08
Forfeited		(39,600	)	$	9.96
Vested		—		$	—
Unvested restricted stock units as of June 30, 2017		408,400		$	10.01

Summary of Stock Based Compensation Expense

	Three Months Ended June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
Research and development expenses	$	2,094	$	2,896	$	3,967	$	5,082
General and administrative expenses		2,409		1,821		4,814		3,539
	$	4,503	$	4,717	$	8,781	$	8,621

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands

Jun. 30, 2017

Dec. 31, 2016

Assets:

Cash Equivalents

$ 3,317

$ 6,307

Repurchase Agreements

7,000

Investments:

Total assets

144,849

188,763

Fair Value, Inputs, Level 2 [Member]

Assets:

Cash Equivalents

1,000

4,740

Repurchase Agreements

7,000

Investments:

Total assets

142,532

187,196

Not Subject to Leveling [Member]

Assets:

Cash Equivalents

^[1]

2,317

1,567

Investments:

Total assets

^[1]

2,317

1,567

Commercial Paper [Member]

Investments:

Investments

16,699

19,689

Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]

Investments:

Investments

16,699

19,689

Certificates of Deposit [Member]

Investments:

Investments

8,174

10,629

Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]

Investments:

Investments

8,174

10,629

Corporate Bonds [Member]

Investments:

Investments

74,174

94,609

Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]

Investments:

Investments

74,174

94,609

Government Securities [Member]

Investments:

Investments

26,431

33,466

Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]

Investments:

Investments

26,431

33,466

Treasury Bonds [Member]

Investments:

Investments

16,054

17,063

Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]

Investments:

Investments

$ 16,054

$ 17,063

[1]	Certain cash equivalents and investments that are valued using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy.

Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 141,707	$ 175,603
Gross Unrealized Gain		59
Gross Unrealized Loss	(175)	(206)
Fair Value	141,532	175,456
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,705	19,631
Gross Unrealized Gain		58
Gross Unrealized Loss	(6)
Fair Value	16,699	19,689
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,174	10,629
Fair Value	8,174	10,629
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	74,261	94,764
Gross Unrealized Loss	(87)	(155)
Fair Value	74,174	94,609
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,500	33,513
Gross Unrealized Loss	(69)	(47)
Fair Value	26,431	33,466
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,067	17,066
Gross Unrealized Gain		1
Gross Unrealized Loss	(13)	(4)
Fair Value	$ 16,054	$ 17,063

Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Equity [Abstract]
Number of Shares, Beginning Balance	5,069,133
Number of Shares, Granted	1,609,500
Number of Shares, Exercised	(56,282)
Number of Shares, Forfeited	(307,513)
Number of Shares, Ending Balance	6,314,838	5,069,133
Number of Shares, Options exercisable	2,800,162
Weighted Average Exercise Price, Beginning Balance	$ 14.36
Weighted Average Exercise Price, Granted	10.07
Weighted Average Exercise Price, Exercised	0.65
Weighted Average Exercise Price, Forfeited	20.18
Weighted Average Exercise Price, Ending Balance	13.11	$ 14.36
Weighted Average Exercise Price, Options exercisable	$ 11.10
Weighted Average Remaining Contractual Term, Outstanding	8 years 1 month 24 days	8 years 3 months 4 days
Weighted Average Remaining Contractual Term, Options exercisable	7 years 4 months 2 days
Aggregate Intrinsic Value, Outstanding	$ 20,875	$ 16,736
Aggregate Intrinsic Value, Options exercisable	$ 14,634

Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) - Restricted Stock Units [Member]	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Unvested restricted stock units, Beginning balance \| shares	115,500
Number of Shares, Granted \| shares	332,500
Number of Shares, Forfeited \| shares	(39,600)
Number of Shares, Unvested restricted stock units, Ending balance \| shares	408,400
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Beginning balance \| $ / shares	$ 9.78
Weighted Average Grant Date Fair Value, Granted \| $ / shares	10.08
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	9.96
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance \| $ / shares	$ 10.01

Collaboration Revenue - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Period of performance identifiable deliverables		10 years
Collaboration revenue - related party			$ 3,014	$ 3,004	$ 6,029	$ 5,714
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 120,000				120,000
Development milestone payments		$ 285,000
Regulatory payments		375,000
Sales based milestones		$ 1,125,000
Reimbursement of development services					580
Proceeds on achievement of development milestone							$ 10,000
Collaboration revenue associated with substantive milestones - related party							$ 10,000
Collaboration revenue - related party			3,014	$ 3,004	6,029	$ 5,714
Deferred revenue			$ 102,785		$ 102,785

Commitments and Contingencies - Additional Information (Detail)		3 Months Ended		6 Months Ended
	Nov. 11, 2015 USD ($) ft²	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Indemnification Agreement [Member]
Other Commitments [Line Items]
Indemnification obligations accrued		$ 0		$ 0		$ 0
Office and Laboratory Space [Member]
Other Commitments [Line Items]
Rental expense related to office and laboratory space		1,145,000	$ 787,000	$ 2,244,000	$ 1,180,000
BMR Sidney Research Campus LLC [Member]
Other Commitments [Line Items]
Non-cancelable property lease, description				On November 11, 2015, the Company entered into a non-cancelable property lease with BMR-Sidney Research Campus LLC (“BMR”) for 83,396 square feet of office, laboratory and pilot manufacturing space at 200 Sidney Street, Cambridge, Massachusetts. The lease term commenced in March 2016 and ends in November 2023. The Company has the option to extend the lease twice, each for a five-year period. The Company moved its corporate headquarters to this location in April 2016
Square feet of property subject to non-cancelable lease \| ft²	83,396
Lease commencement period				2016-03
Lease expiration period				2023-11
Cost of tenant improvements	$ 12,509,000
Lease incentive obligation		$ 11,196,000		$ 11,196,000
Amortized of lease incentive obligation				$ 884,000

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 3,014,000	$ 3,004,000	$ 6,029,000	$ 5,714,000
Flagship Ventures Management INC [Member] \| Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0	1,000	0	17,000
Due to related party	0		0		$ 0
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0		0
Collaboration revenue - related party	3,014,000	$ 3,004,000	6,029,000	$ 5,714,000
Deferred revenue	102,785,000		102,785,000
Due from related party for the reimbursement of development costs	$ 297,000		$ 297,000

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated year	Oct. 31, 2010
Entity incorporated state name	State of Delaware
Accumulated deficit	$ 227,683	$ 174,191
Cash, cash equivalents and investments	$ 175,174

Investments - Additional Information (Detail)	6 Months Ended
Investments - Additional Information (Detail)	Jun. 30, 2017
Investments [Abstract]
Maximum maturity days for cash equivalents	90 days

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 43,484	$ 41,305
Less: Accumulated depreciation and amortization	(8,671)	(5,180)
Property and equipment, net	34,813	36,125
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	11,802	10,711
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,366	1,335
Furniture and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,033	1,010
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,896	27,807
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,387	$ 442

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development and manufacturing costs	$ 3,763	$ 3,350
Payroll and payroll-related costs	2,707	3,698
Professional fees	505	448
Facility and other	2,743	3,316
Total Accrued Expenses and Other Current Liabilities	$ 9,718	$ 10,812

Subsequent Events - Additional Information (Detail) $ in Thousands	1 Months Ended
	Jul. 31, 2017 USD ($)
Subsequent Event [Member] \| Phase 3 Clinical Trials [Member]
Subsequent Event [Line Items]
Revenue recorded upon achievement of related milestone	$ 20,000