Seres Therapeutics (MCRB) 10-Q 8 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Nov. 03, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	MCRB
Entity Registrant Name	Seres Therapeutics, Inc.
Entity Central Index Key	1,609,809
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		40,530,139

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 46,025	$ 54,539
Investments	125,274	138,704
Prepaid expenses and other current assets	5,346	5,126
Total current assets	176,645	198,369
Property and equipment, net	33,724	36,125
Long-term investments		36,752
Restricted cash	1,513	1,400
Total assets	211,882	272,646
Current liabilities:
Accounts payable	5,380	7,587
Accrued expenses and other current liabilities	9,594	10,812
Deferred revenue - related party	12,058	12,058
Total current liabilities	27,032	30,457
Lease incentive obligation, net of current portion	9,424	10,730
Deferred rent	2,202	2,072
Deferred revenue, net of current portion - related party	87,712	96,756
Total liabilities	126,370	140,015
Commitments and contingencies (Note 11)
Stockholders’ equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at September 30, 2017 and December 31, 2016; no shares issued and outstanding at September 30, 2017 and December 31, 2016
Common stock, $0.001 par value; 200,000,000 shares authorized at September 30, 2017 and December 31, 2016; 40,512,639 and 40,355,753 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	40	40
Additional paid-in capital	320,189	306,931
Accumulated other comprehensive loss	(99)	(149)
Accumulated deficit	(234,618)	(174,191)
Total stockholders’ equity	85,512	132,631
Total liabilities and stockholders’ equity	$ 211,882	$ 272,646

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 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	40,512,639	40,355,753
Common stock, shares outstanding	40,512,639	40,355,753

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue:
Collaboration revenue - related party	$ 23,015	$ 13,015	$ 29,044	$ 18,730
Total revenue	23,015	13,015	29,044	18,730
Operating expenses:
Research and development expenses	22,210	24,143	65,413	61,733
General and administrative expenses	8,119	7,967	25,251	24,163
Total operating expenses	30,329	32,110	90,664	85,896
Loss from operations	(7,314)	(19,095)	(61,620)	(67,166)
Other income (expense):
Interest income	502	719	1,892	1,483
Other income (expense)	(123)	(312)	(699)	(620)
Total other income, net	379	407	1,193	863
Net loss	$ (6,935)	$ (18,688)	$ (60,427)	$ (66,303)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.17)	$ (0.46)	$ (1.49)	$ (1.67)
Weighted average common shares outstanding, basic and diluted	40,494,049	40,235,623	40,419,522	39,676,085
Other comprehensive (loss) income:
Unrealized (loss) gain on investments, net of tax of $0	$ 77	$ (150)	$ 50	$ (97)
Total other comprehensive (loss) income	77	(150)	50	(97)
Comprehensive loss	$ (6,858)	$ (18,838)	$ (60,377)	$ (66,400)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Unrealized (loss) gain on investment, tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (60,427)	$ (66,303)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	13,150	12,865
Depreciation and amortization expense	5,356	2,511
Non-cash interest expense	3	2
Accretion of discount on investments	(166)	(317)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(220)	(2,527)
Deferred revenue	(9,044)	111,545
Accounts payable	(2,502)	84
Accrued expenses and other current liabilities	(1,065)	6,866
Net cash provided by (used in) operating activities	(54,915)	64,726
Cash flows from investing activities:
Purchases of property and equipment	(3,991)	(15,805)
Purchases of investments	(75,728)	(245,728)
Sales and maturities of investments	126,125	176,230
Changes in restricted cash	(113)	117
Net cash provided by (used in) investing activities	46,293	(85,186)
Cash flows from financing activities:
Proceeds from exercise of stock options and common stock warrants	108	2,138
Net cash provided by financing activities	108	2,138
Net decrease in cash and cash equivalents	(8,514)	(18,322)
Cash and cash equivalents at beginning of period	54,539	73,933
Cash and cash equivalents at end of period	46,025	55,611
Supplemental disclosure of cash flow information:
Cash paid for interest	1	109
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 453	$ 4,484

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 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 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 $234,618 and $174,191 as of September 30, 2017 and December 31, 2016, respectively. The Company expects that its cash, cash equivalents and investments at September 30, 2017 of $171,299 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 September 30, 2017 and for the three and nine months ended September 30, 2017 and 2016 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 September 30, 2017 and consolidated results of operations for the three and nine months ended September 30, 2017 and its cash flows for the nine months ended September 30, 2017 and 2016. Such adjustments are of a normal and recurring nature. The results of operations for the three and nine months ended September 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 Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 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 nine months ended September 30, 2017. During the three and nine months ended September 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 Nine Months Ended September 30, 2017 2016 Stock options to purchase common stock 6,133,596 5,165,729 Unvested restricted stock units 401,900 — 6,535,496 5,165,729 Recently Issued Accounting Standards In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients The Company plans to adopt ASC 606 using the modified retrospective transition method, which will result in an adjustment to accumulated deficit in its consolidated balance sheet as of the January 1, 2018 effective date for the cumulative effect of applying the standard. As the adoption method does not result in a recast of the prior year consolidated financial statements, ASC 606 requires the Company to provide additional disclosures during the year of adoption of the amount by which each financial statement line item is affected by adoption of the new standard and explanations of the reasons for significant changes. The Company is currently evaluating the impact of the adoption of ASC 606 on its consolidated financial statements. While its assessment is preliminary, the Company expects the adoption will change the pattern and timing of revenue recognition of amounts from its collaboration agreement with NHS. Under ASC 606, the Company will recognize revenue using the cost-to-cost method which best depicts the transfer of control to the customer. In contrast, under existing revenue recognition standard, the Company is recognizing revenue on a straight-line basis over the estimated period of performance. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimate of the Company’s measure of progress and estimate of transaction price will be updated at each reporting date, as a change in estimate, and will require judgement. The amount of consideration allocated to satisfied performance obligations, based on the Company’s measure of progress will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time. The Company is in process with calculating the revenue recognition impact under the cost-to-cost method, and the amounts that will be recognized upon adoption on January 1, 2018. 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

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 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 following table presents information about the Company’s assets as of September 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 September 30, 2017 Using: Level 1 Level 2 Level 3 Not Subject to Leveling (1) Total Assets: Cash Equivalents $ — $ 3,000 $ — $ 11,237 $ 14,237 Investments: Commercial Paper $ — $ 9,737 $ — $ — $ 9,737 Certificates of Deposit — 10,142 — — 10,142 Corporate Bonds — 70,923 — — 70,923 Government Securities — 24,952 — — 24,952 Treasury Bonds — 9,520 — — 9,520 $ — $ 128,274 $ — $ 11,237 $ 139,511 (1) 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) As of September 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 September 30, 2017 and December 31, 2016 were determined using Level 2 inputs. During the three and nine months ended September 30, 2017 and 2016 there were no transfers between Level 1, Level 2 and Level 3.

Investments

Investments	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Investments	4. Investments As of September 30, 2017 and December 31, 2016, the fair value of available-for-sale investments by type of security was as follows: September 30, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 9,738 $ — $ (1 ) $ 9,737 Certificates of Deposit 10,142 — — 10,142 Corporate Bonds 70,967 1 (45 ) 70,923 Government Securities 24,995 — (43 ) 24,952 Treasury Bonds 9,529 — (9 ) 9,520 $ 125,371 $ 1 $ (98 ) $ 125,274 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

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following: September 30, 2017 December 31, 2016 Laboratory equipment $ 12,544 $ 10,711 Computer equipment 2,555 1,335 Furniture and office equipment 1,033 1,010 Leasehold improvements 27,896 27,807 Construction in progress 232 442 44,260 41,305 Less: Accumulated depreciation and amortization (10,536 ) (5,180 ) $ 33,724 $ 36,125 Depreciation and amortization expense was $1,866, $5,356, $1,251, and $2,511 for the three and nine months ended September 30, 2017 and 2016, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2017
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: September 30, 2017 December 31, 2016 Development and manufacturing costs $ 2,987 $ 3,350 Payroll and payroll-related costs 3,302 3,698 Professional fees 490 448 Facility and other 2,815 3,316 $ 9,594 $ 10,812

Preferred Stock

Preferred Stock	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Preferred Stock	7. Preferred Stock On July 1, 2015, in connection with the closing of the Company’s initial public offering of its common stock (the “IPO”), the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 10,000,000 shares of preferred stock, $0.001 par value per share.

Stockholders' Equity Common Sto

Stockholders' Equity Common Stock	9 Months Ended
	Sep. 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 1,696,000 10.19 Exercised (156,886 ) 0.69 Forfeited (474,651 ) 20.66 Outstanding as of September 30, 2017 6,133,596 $ 13.07 7.92 $ 36,791 Options exercisable as of September 30, 2017 2,973,186 $ 11.32 7.14 $ 22,602 The weighted average grant-date fair value of stock options granted during the three and nine months ended September 30, 2017 and 2016 was $8.43, $7.06, $12.45, and $18.93 per share, respectively. Restricted Stock The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the nine months ended September 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 (46,100 ) $ 9.93 Vested — $ — Unvested restricted stock units as of September 30, 2017 401,900 $ 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 September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development expenses $ 2,158 $ 2,334 $ 6,124 $ 7,416 General and administrative expenses 2,211 1,910 7,026 5,449 $ 4,369 $ 4,244 $ 13,150 $ 12,865

Collaboration Revenue

Collaboration Revenue	9 Months Ended
	Sep. 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 Collaboration and License Agreement (“License Agreement”) with Nestec Ltd. (“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 IBD, including UC 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 September 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 Revenue Recognition-Milestone Method During the three months ended September 30, 2017, the Company received $20,000 from NHS in connection with the initiation of the Phase 3 study for SER-109. The Company recognizes revenue associated with substantive milestones in accordance with FASB ASC Topic 605-28, Revenue Recognition-Milestone Method Royalties will be recorded as revenue in the period they are earned assuming all other revenue recognition criteria are met. During the three and nine months ended September 30, 2017 and 2016, the Company recognized $23,015, $29,044, $13,015, and $18,730, respectively, of related party revenue (see Note 12 “Related Party Transactions”) associated with the License Agreement. As of September 30, 2017, there was $99,770 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.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The Company did not provide for any income taxes for the nine-month period ended September 30, 2017 or 2016. The Company has evaluated the positive and negative evidence bearing upon the realizability of its U.S. net deferred tax assets. As required by the provisions of ASC 740, Income Taxes As of September 30, 2017 and December 31, 2016, the Company had no accrued interest or tax penalties recorded. The Company files income tax returns in the U.S. 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 my 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

Commitments and Contingencies	9 Months Ended
	Sep. 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 September 30, 2017, the Company has recorded a lease incentive obligation of $10,754. During the nine months ended September 30, 2017, we amortized $1,326 of this lease incentive obligation as a reduction to rent expense. During the three and nine months ended September 30, 2017 and 2016, the Company recognized $1,100, $3,345, $1,406, and $2,587 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 September 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.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions In October 2010, the Company entered into a services agreement with Flagship Ventures Management, Inc., an affiliate of one of its stockholders, Flagship Venture Funds (and now known as Flagship Pioneering), to provide general and administrative services to the Company, including the employer portions of employee health and dental benefit plans for Seres Therapeutics employees and consulting services. The Company made payments under the agreement of $17 during the nine months ended September 30, 2016. There were no payments made during the three and nine months ended September 30, 2017 and the three months ended September 30, 2016. T here were no amounts due to Flagship Ventures Management, Inc. related to the services agreement as of September 30, 2017 or December 31, 2016. As described in Note 9, in January 2016 the Company entered into a 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 nine months ended September 30, 2017 and 2016, the Company recognized $23,015, $29,044, $13,015, and $18,730 of related party revenue associated with the License Agreement. As of September 30, 2017, there was $99,770 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 nine months ended September 30, 2017. There is $297 due from NHS as of September 30, 2017 for the reimbursement of development costs, which is classified as other current assets in the Company’s consolidated balance sheet.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events In October, 2017 the Combating Antibiotic Resistant Bacteria Biopharmaceutical Accelerator ("CARB-X”), an international public-private partnership focused on advancing new antimicrobial products to address the threat of antibiotic resistance, selected the Company to receive up to $2,461 in research funding for SER-155, with potential for an additional $3,119 upon the completion of milestones. In connection with this funding, the Company entered into a cost reimbursement Sub-Award Agreement (the “Sub-award Agreement”) with the Trustees of Boston University, the administrator of the program. The Company expects to begin incurring reimbursable expenses under the Sub-Award Agreement in November 2017.

Summary of Significant Accoun20

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition, the accrual of research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Net Loss per Share	Net Loss per Share Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period and, if dilutive, the weighted average number of potential shares of common stock, including the assumed exercise of stock options and 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 Nine Months Ended September 30, 2017 2016 Stock options to purchase common stock 6,133,596 5,165,729 Unvested restricted stock units 401,900 — 6,535,496 5,165,729
Recently Issued Accounting Standards	Recently Issued Accounting Standards In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients The Company plans to adopt ASC 606 using the modified retrospective transition method, which will result in an adjustment to accumulated deficit in its consolidated balance sheet as of the January 1, 2018 effective date for the cumulative effect of applying the standard. As the adoption method does not result in a recast of the prior year consolidated financial statements, ASC 606 requires the Company to provide additional disclosures during the year of adoption of the amount by which each financial statement line item is affected by adoption of the new standard and explanations of the reasons for significant changes. The Company is currently evaluating the impact of the adoption of ASC 606 on its consolidated financial statements. While its assessment is preliminary, the Company expects the adoption will change the pattern and timing of revenue recognition of amounts from its collaboration agreement with NHS. Under ASC 606, the Company will recognize revenue using the cost-to-cost method which best depicts the transfer of control to the customer. In contrast, under existing revenue recognition standard, the Company is recognizing revenue on a straight-line basis over the estimated period of performance. Under the cost-to-cost method, the extent of progress towards completion is measured based on the ratio of actual costs incurred to the total estimated costs expected upon satisfying the identified performance obligation. Under this method, revenue will be recorded as a percentage of the estimated transaction price based on the extent of progress towards completion. The estimate of the Company’s measure of progress and estimate of transaction price will be updated at each reporting date, as a change in estimate, and will require judgement. The amount of consideration allocated to satisfied performance obligations, based on the Company’s measure of progress will be recognized immediately on a cumulative catch-up basis, resulting in an adjustment to revenue in the period of change. The amount related to the unsatisfied portion will be recognized as that portion is satisfied over time. The Company is in process with calculating the revenue recognition impact under the cost-to-cost method, and the amounts that will be recognized upon adoption on January 1, 2018. 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

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share	The following potential common shares, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three and Nine Months Ended September 30, 2017 2016 Stock options to purchase common stock 6,133,596 5,165,729 Unvested restricted stock units 401,900 — 6,535,496 5,165,729

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table presents information about the Company’s assets as of September 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 September 30, 2017 Using: Level 1 Level 2 Level 3 Not Subject to Leveling (1) Total Assets: Cash Equivalents $ — $ 3,000 $ — $ 11,237 $ 14,237 Investments: Commercial Paper $ — $ 9,737 $ — $ — $ 9,737 Certificates of Deposit — 10,142 — — 10,142 Corporate Bonds — 70,923 — — 70,923 Government Securities — 24,952 — — 24,952 Treasury Bonds — 9,520 — — 9,520 $ — $ 128,274 $ — $ 11,237 $ 139,511 (1) 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)

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Schedule of Fair Value of Available-for-Sale Investments by Type of Security	As of September 30, 2017 and December 31, 2016, the fair value of available-for-sale investments by type of security was as follows: September 30, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value Investments: Commercial Paper $ 9,738 $ — $ (1 ) $ 9,737 Certificates of Deposit 10,142 — — 10,142 Corporate Bonds 70,967 1 (45 ) 70,923 Government Securities 24,995 — (43 ) 24,952 Treasury Bonds 9,529 — (9 ) 9,520 $ 125,371 $ 1 $ (98 ) $ 125,274 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

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following: September 30, 2017 December 31, 2016 Laboratory equipment $ 12,544 $ 10,711 Computer equipment 2,555 1,335 Furniture and office equipment 1,033 1,010 Leasehold improvements 27,896 27,807 Construction in progress 232 442 44,260 41,305 Less: Accumulated depreciation and amortization (10,536 ) (5,180 ) $ 33,724 $ 36,125

Accrued Expenses and Other Cu25

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
Payables And Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: September 30, 2017 December 31, 2016 Development and manufacturing costs $ 2,987 $ 3,350 Payroll and payroll-related costs 3,302 3,698 Professional fees 490 448 Facility and other 2,815 3,316 $ 9,594 $ 10,812

Stockholders' Equity Common S26

Stockholders' Equity Common Stock (Tables)	9 Months Ended
	Sep. 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 1,696,000 10.19 Exercised (156,886 ) 0.69 Forfeited (474,651 ) 20.66 Outstanding as of September 30, 2017 6,133,596 $ 13.07 7.92 $ 36,791 Options exercisable as of September 30, 2017 2,973,186 $ 11.32 7.14 $ 22,602
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 nine months ended September 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 (46,100 ) $ 9.93 Vested — $ — Unvested restricted stock units as of September 30, 2017 401,900 $ 10.01
Summary of Stock Based Compensation Expense	The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its condensed consolidated statements of operations and comprehensive loss: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development expenses $ 2,158 $ 2,334 $ 6,124 $ 7,416 General and administrative expenses 2,211 1,910 7,026 5,449 $ 4,369 $ 4,244 $ 13,150 $ 12,865

Nature of the Business and Ba27

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 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	$ 234,618	$ 174,191
Cash, cash equivalents and investments	$ 171,299

Summary of Significant Accoun28

Summary of Significant Accounting Policies - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,535,496	5,165,729	6,535,496	5,165,729
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	6,133,596	5,165,729	6,133,596	5,165,729
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from calculation of diluted net loss per share	401,900		401,900

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair value of liabilities on recurring basis	$ 0		$ 0		$ 0
Fair value, assets transfers from Level 1 to Level 2 measurement	0	$ 0	0	$ 0
Fair value, assets transfers from Level 2 to Level 1 measurement	0	0	0	0
Fair Value, assets transfers into (out of) Level 3 measurement	$ 0	$ 0	$ 0	$ 0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Assets:
Cash Equivalents		$ 14,237	$ 6,307
Repurchase Agreements			7,000
Investments:
Total assets		139,511	188,763
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash Equivalents		3,000	4,740
Repurchase Agreements			7,000
Investments:
Total assets		128,274	187,196
Not Subject to Leveling [Member]
Assets:
Cash Equivalents	[1]	11,237	1,567
Investments:
Total assets	[1]	11,237	1,567
Commercial Paper [Member]
Investments:
Investments		9,737	19,689
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		9,737	19,689
Certificates of Deposit [Member]
Investments:
Investments		10,142	10,629
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		10,142	10,629
Corporate Bonds [Member]
Investments:
Investments		70,923	94,609
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		70,923	94,609
Government Securities [Member]
Investments:
Investments		24,952	33,466
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		24,952	33,466
Treasury Bonds [Member]
Investments:
Investments		9,520	17,063
Treasury Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Investments:
Investments		$ 9,520	$ 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

Investments - Schedule of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 125,371	$ 175,603
Gross Unrealized Gain	1	59
Gross Unrealized Loss	(98)	(206)
Fair Value	125,274	175,456
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,738	19,631
Gross Unrealized Gain		58
Gross Unrealized Loss	(1)
Fair Value	9,737	19,689
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,142	10,629
Fair Value	10,142	10,629
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	70,967	94,764
Gross Unrealized Gain	1
Gross Unrealized Loss	(45)	(155)
Fair Value	70,923	94,609
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,995	33,513
Gross Unrealized Loss	(43)	(47)
Fair Value	24,952	33,466
Treasury Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,529	17,066
Gross Unrealized Gain		1
Gross Unrealized Loss	(9)	(4)
Fair Value	$ 9,520	$ 17,063

Investments - Additional Inform

Investments - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017
Investments [Abstract]
Maximum maturity days for cash equivalents	90 days

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 44,260	$ 41,305
Less: Accumulated depreciation and amortization	(10,536)	(5,180)
Property and equipment, net	33,724	36,125
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	12,544	10,711
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,555	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	$ 232	$ 442

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,866	$ 1,251	$ 5,356	$ 2,511

Accrued Expenses and Other Cu35

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development and manufacturing costs	$ 2,987	$ 3,350
Payroll and payroll-related costs	3,302	3,698
Professional fees	490	448
Facility and other	2,815	3,316
Total Accrued Expenses and Other Current Liabilities	$ 9,594	$ 10,812

Preferred Stock - Additional In

Preferred Stock - Additional Information (Detail) - $ / shares	Sep. 30, 2017	Dec. 31, 2016	Jul. 01, 2015
Equity [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001	$ 0.001

Stockholders' Equity Common S37

Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Equity [Abstract]
Number of Shares, Beginning Balance	5,069,133
Number of Shares, Granted	1,696,000
Number of Shares, Exercised	(156,886)
Number of Shares, Forfeited	(474,651)
Number of Shares, Ending Balance	6,133,596	5,069,133
Number of Shares, Options exercisable	2,973,186
Weighted Average Exercise Price, Beginning Balance	$ 14.36
Weighted Average Exercise Price, Granted	10.19
Weighted Average Exercise Price, Exercised	0.69
Weighted Average Exercise Price, Forfeited	20.66
Weighted Average Exercise Price, Ending Balance	13.07	$ 14.36
Weighted Average Exercise Price, Options exercisable	$ 11.32
Weighted Average Remaining Contractual Term, Outstanding	7 years 11 months 1 day	8 years 3 months 4 days
Weighted Average Remaining Contractual Term, Options exercisable	7 years 1 month 21 days
Aggregate Intrinsic Value, Outstanding	$ 36,791	$ 16,736
Aggregate Intrinsic Value, Options exercisable	$ 22,602

Stockholders' Equity Common S38

Stockholders' Equity Common Stock - Stock Options - Additional Information (Detail) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Weighted average grant-date fair value of stock options	$ 8.43	$ 12.45	$ 7.06	$ 18.93

Stockholders' Equity Common S39

Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) - Restricted Stock Units [Member]	9 Months Ended
	Sep. 30, 2017$ / sharesshares
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	(46,100)
Number of Shares, Unvested restricted stock units, Ending balance | shares	401,900
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	9.93
Weighted Average Grant Date Fair Value, Unvested restricted stock units, Ending balance | $ / shares	$ 10.01

Stockholders' Equity Common S40

Stockholders' Equity Common Stock - Summary of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,369	$ 4,244	$ 13,150	$ 12,865
Research and development expenses [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,158	2,334	6,124	7,416
General and administrative expenses [Member] | Stock Options and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,211	$ 1,910	$ 7,026	$ 5,449

Collaboration Revenue - Additio

Collaboration Revenue - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Period of performance identifiable deliverables		10 years
Collaboration revenue - related party			$ 23,015	$ 13,015	$ 29,044	$ 18,730
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			20,000				$ 10,000
Collaboration revenue associated with substantive milestones - related party			20,000				$ 10,000
Collaboration revenue - related party			23,015	$ 13,015	29,044	$ 18,730
Deferred revenue			$ 99,770		$ 99,770

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 0	$ 0
Accrued interest or tax penalties	$ 0		$ 0

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Nov. 11, 2015USD ($)ft²	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
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		1,100,000	$ 1,406,000	$ 3,345,000	$ 2,587,000
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		$ 10,754,000		$ 10,754,000
Amortized of lease incentive obligation				$ 1,326,000

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party	$ 23,015,000	$ 13,015,000	$ 29,044,000	$ 18,730,000
Flagship Ventures Management INC [Member] | Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party	0	0	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	23,015,000	$ 13,015,000	29,044,000	$ 18,730,000
Deferred revenue	99,770,000		99,770,000
Due from related party for the reimbursement of development costs	$ 297,000		$ 297,000

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - CARB-X [Member] - Subsequent Event [Member]	1 Months Ended
	Oct. 31, 2017USD ($)
Subsequent Event [Line Items]
Maximum amount of research funding receivable	$ 2,461,000
Additional research funding receivable based on completion of milestones	$ 3,119,000
Reimbursable expense, beginning period	2017-11