Aimmune Therapeutics (AIMT) 10-Q 8 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	AIMT
Entity Registrant Name	AIMMUNE THERAPEUTICS, INC.
Entity Central Index Key	1,631,650
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		58,730,708

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 78,420	$ 73,487
Short-term investments	176,829	108,943
Prepaid expenses and other current assets	6,543	6,681
Total current assets	261,792	189,111
Property and equipment, net	25,045	17,205
Prepaid expenses and other assets	626	618
Total assets	287,463	206,934
Current liabilities:
Accounts payable	13,890	5,095
Accrued liabilities	23,545	21,478
Other current liabilities	16	26
Total current liabilities	37,451	26,599
Other liabilities	2,848	2,530
Total liabilities	40,299	29,129
Commitments and contingencies (Note 5)
Stockholders’ equity:
Common stock, par value $0.0001 per share—290,000 shares authorized as of September 30, 2018, and December 31, 2017; 58,651 and 51,091 shares issued and outstanding as of September 30, 2018, and December 31, 2017, respectively (including 0 and 47 shares subject to repurchase, legally issued and outstanding as of September 30, 2018, and December 31, 2017, respectively)	6	5
Additional paid-in capital	666,532	443,390
Accumulated other comprehensive loss	(138)	(108)
Accumulated deficit	(419,236)	(265,482)
Total stockholders’ equity	247,164	177,805
Total liabilities and stockholders’ equity	$ 287,463	$ 206,934

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	290,000,000	290,000,000
Common stock, shares issued	58,651,000	51,091,000
Common stock, shares outstanding	58,651,000	51,091,000
Common Stock Subject to Repurchase
Common stock, shares issued	0	47,000
Common stock, shares outstanding	0	47,000

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses
Research and development	$ 31,691	$ 21,063	$ 100,391	$ 60,671
General and administrative	21,285	11,226	56,517	30,963
Total operating expenses	52,976	32,289	156,908	91,634
Loss from operations	(52,976)	(32,289)	(156,908)	(91,634)
Interest income, net	1,303	497	3,233	1,475
Loss before provision for income taxes	(51,673)	(31,792)	(153,675)	(90,159)
Provision for income taxes	29		79
Net loss	(51,702)	(31,792)	(153,754)	(90,159)
Other comprehensive loss, net of tax:
Unrealized gains (losses) on investments	(15)	61	(30)	(47)
Comprehensive loss	$ (51,717)	$ (31,731)	$ (153,784)	$ (90,206)
Net loss per common share, basic and diluted	$ (0.89)	$ (0.63)	$ (2.72)	$ (1.79)
Weighted average shares used in computing net loss per common share, basic and diluted	58,274	50,458	56,602	50,254

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (153,754)	$ (90,159)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation expense	1,121	703
Stock-based compensation expense	24,663	11,871
Amortization of premium on investment securities	(529)	477
Changes in operating assets and liabilities:
Prepaid expenses and other assets	412	(835)
Accounts payable	8,133	2,874
Accrued liabilities	2,067	6,927
Other liabilities	309	464
Net cash used in operating activities	(117,578)	(67,678)
Cash flows from investing activities:
Purchase of property and equipment	(8,300)	(5,252)
Purchase of investments	(192,285)	(109,391)
Maturities of investments	124,898	129,180
Net cash provided by (used in) investing activities	(75,687)	14,537
Cash flows from financing activities:
Proceeds from underwritten public offering, net of offering costs	190,435
Net cash proceeds from exercise of stock options, including early exercise	7,763	2,952
Net cash provided by financing activities	198,198	2,952
Net increase (decrease) in cash and cash equivalents	4,933	(50,189)
Cash and cash equivalents at the beginning of the period	73,487	124,010
Cash and cash equivalents at the end of the period	78,420	$ 73,821
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued liabilities	660
Receivable for stock option exercises	$ (281)

Formation and Business of the C

Formation and Business of the Company	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Formation and Business of the Company	1. Formation and Business of the Company Aimmune Therapeutics, Inc., or the Company, is a clinical-stage biopharmaceutical company advancing a new therapeutic approach, including the development of proprietary product candidates, for the treatment of peanut and other food allergies. Our therapeutic approach, which we refer to as Characterized Oral Desensitization Immunotherapy, or CODIT TM Since inception, we have incurred net losses and negative cash flows from operations. During the quarter and nine months ended September 30, 2018, we incurred a net loss of $51.7 million and $153.8 million, respectively, and during the nine months ended September 30, 2018, we used $117.6 million of cash in operations. As of September 30, 2018, we had an accumulated deficit of $419.2 million, and we do not expect to experience positive cash flows in the near future. As of September 30, 2018, we had cash, cash equivalents and investments of $255.2 million. We believe that our existing capital resources will be sufficient to fund our planned operations for at least the next 12 months and through expected regulatory approval in the United States for AR101, our lead CODIT TM

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Preparation The accompanying condensed consolidated financial statements have been prepared in accordance with Generally Accepted Accounting Principles, or GAAP, in the United States and applicable rules and regulations of the Securities and Exchange Commission, or SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2017, has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These condensed consolidated financial statements have been prepared on the same basis as our annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of our financial information. The results of operations for the quarter and nine months ended September 30, 2018, are not necessarily indicative of the results to be expected for the year ending December 31, 2018, or for any other interim period or for any other future year. We operate in one reportable segment. The accompanying condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2017, included in our Annual Report on Form 10-K filed with the SEC. Basis of Consolidation The accompanying condensed consolidated financial statements include the accounts of our wholly-owned subsidiaries. All significant intercompany transactions have been eliminated. Use of Estimates The preparation of the accompanying condensed consolidated financial statements in accordance with U.S. GAAP requires us 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 condensed consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions. Significant Accounting Policies There have been no significant changes to the accounting policies during the quarter and nine months ended September 30, 2018, as compared to the significant accounting policies described in Note 2 of the “Notes to Consolidated Financial Statements” in our audited consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2017, except as noted below. Recently Adopted Accounting Pronouncements In May 2017, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting We adopted ASU 2017-09 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows – Restricted Cash (Topic 230) We adopted ASU 2016-18 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. In October 2016, the FASB, issued ASU, 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers Other Than Inventory We adopted ASU 2016-16 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments We adopted ASU 2016-15 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued Accounting Standards Update No. 2018-13, Fair Value Measurement: Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In March 2017, the FASB issued ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities We are currently evaluating the impact that the adoption of ASU 2017-08 will have on our consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In February 2016, the FASB, issued ASU, No. 2016-02, Leases (Topic 842) Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements

Available-for-Sale Securities a

Available-for-Sale Securities and Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Available-for-Sale Securities and Fair Value Measurements	3. Available-for-Sale Securities and Fair Value Measurements We define fair value 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 Our valuation techniques are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect our market assumptions. We classify these inputs into the following hierarchy: • Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; • Level 2—Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and • Level 3—Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The following tables set forth our financial instrumen ts that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 75,921 $ — $ — $ 75,921 U.S. government securities — 2,499 — $ 2,499 Total cash and cash equivalents $ 75,921 $ 2,499 $ — $ 78,420 Investments: Agency securities $ — $ 16,899 $ — $ 16,899 Corporate securities — 49,099 — 49,099 Commercial paper — 3,999 — 3,999 U.S. government securities — 106,832 — 106,832 Total investments $ — $ 176,829 $ — $ 176,829 December 31, 2017 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 39,072 $ — $ — $ 39,072 Corporate securities — 999 — 999 Commercial paper — 33,416 — 33,416 Total cash and cash equivalents $ 39,072 $ 34,415 $ — $ 73,487 Investments: Agency securities — 12,718 — 12,718 Corporate securities — 28,345 — 28,345 Commercial paper — 21,432 — 21,432 U.S. government securities — 46,448 — 46,448 Total investments $ — $ 108,943 $ — $ 108,943 Our valuation techniques used to measure the fair value of money market funds were derived from quoted prices in active markets for identical assets. The valuation techniques used to measure the fair value of investments, all of which have counterparties with high credit ratings, were valued based on quoted market prices or model-driven valuations using significant inputs derived from or corroborated by observable market data. Investments are carried at fair value. During the quarters and nine months ended September 30, 2018 and 2017, there were no transfers between Level 1 and Level 2 of the fair value hierarchy. Available-for-sale investments are carried at fair value and are included in the tables above. The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by security type, classified in cash equivalents and investments, as of September 30, 2018 and December 31, 2017, are as follows September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Total Fair Value Agency securities $ 16,906 $ — $ (7 ) $ 16,899 Corporate securities 49,137 1 (39 ) 49,099 Commercial paper 3,999 — — 3,999 U.S. government securities 109,424 — (93 ) 109,331 Total available-for-sale investments $ 179,466 $ 1 $ (139 ) $ 179,328 December 31, 2017 Amortized Cost Gross unrealized gains Gross unrealized losses Total fair value Agency securities $ 12,729 $ — $ (11 ) $ 12,718 Corporate securities 29,369 1 (26 ) 29,344 Commercial paper 54,848 — — 54,848 U.S. government securities 46,520 — (72 ) 46,448 Total available-for-sale investments $ 143,466 $ 1 $ (109 ) $ 143,358 At September 30, 2018, all of the available-for-sale securities have contractual maturities within twelve months. We periodically review our available-for-sale investments for other-than-temporary impairment loss. We consider factors such as the duration, severity and the reason for the decline in value, the potential recovery period and our intent to sell. For debt securities, we also consider whether (i) it is more likely than not that we will be required to sell the debt securities before recovery of their amortized cost basis, and (ii) the amortized cost basis cannot be recovered as a result of credit losses. During the quarters and nine months ended September 30, 2018 and 2017

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Balance Sheet Components Property and Equipment, Net Property and equipment, net consists of the following (in thousands): September 30, 2018 December 31, 2017 Furniture and equipment $ 2,052 $ 1,655 Computer equipment 1,783 1,410 Manufacturing equipment 885 830 Leased equipment 100 100 Leasehold improvements 2,685 2,685 Buildings 688 688 Construction in progress 19,626 11,490 Property and equipment, gross 27,819 18,858 Less: accumulated depreciation (2,774 ) (1,653 ) Property and equipment, net $ 25,045 $ 17,205 Accrued Liabilities Accrued liabilities consisted of the following (in thousands): September 30, 2018 December 31, 2017 Compensation and benefits $ 7,294 $ 6,205 Research and development 13,203 12,716 Professional and consulting 2,601 2,370 Other 447 187 Total accrued liabilities $ 23,545 $ 21,478

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. Commitments and Contingencies Purchase Commitments We purchase food-grade peanut flour from Golden Peanut Company, or GPC, pursuant to a long-term exclusive commercial supply agreement, which was expanded and extended in January 2018. GPC is precluded from selling several peanut flour products to any third party worldwide for use in oral immunotherapy (OIT) for the treatment or cure of peanut allergy, provided that we are in compliance with our exclusive purchase obligation and meet specified annual purchase commitments. The restated agreement remains in effect until ten years five years. In connection with the expansion and extension of the agreement, we issued Archer Daniels Midland Company 300,000 Pursuant with the restated agreement, our purchase obligation commences with the first delivery of peanut flour for commercial use, which we currently anticipate will occur in 2019. Assuming that our first delivery for commercial use occurs in 2019, which is not assured, the aggregate purchase commitment under this agreement would be $8.3 million over a term of ten years. Indemnifications We indemnify each of our officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as an officer or a director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification is unlimited; however, we currently hold director and officer liability insurance. This insurance allows the transfer of risk associated with our exposure and may enable us to recover a portion of any future amounts paid. We believe that the fair value of these indemnification obligations is minimal. Accordingly, we have not recognized any liabilities relating to these obligations for any period. Legal We are currently not a party to any material legal proceedings. During the normal course of business, we may be a party to legal claims that may not be covered by insurance. We do not believe that any such claims would have a material impact on our consolidated financial statements.

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	6. Stock-Based Compensation Equity Incentive Plan In January 2013, we adopted our Stock Plan (the “2013 Plan”) and in July 2015, we adopted a new Stock Plan (the “2015 Plan”). Upon consummation of our IPO, the 2013 Plan was terminated and no further shares are reserved for issuance under the 2013 Plan. As of September 30, 2018, there were 4.4 million shares reserved for future issuance under our 2015 Plan. As of September 30, 2018, there were 7.3 million shares subject to outstanding options under our 2013 Plan and 2015 Plan. Prior to its termination, the 2013 Plan allowed employees to exercise stock options in exchange for cash before the requisite service was provided (e.g., before the award is vested under its original terms); however, such arrangements permit us to subsequently repurchase such shares at the exercise price if the vesting conditions are not satisfied. Such an exercise is not substantive for accounting purposes. Therefore, the payment received by us for the exercise price is recognized as an early exercise liability on the consolidated balance sheets and will be transferred to common stock and additional paid-in capital as such shares vest. As of September 30, 2018 and Option activity under the 2015 Plan and 2013 Plan is set forth below: Options Outstanding Number Options and Unvested Shares Weighted- Average Exercise Price Weighted Average Remaining Contractual Life (in years) Aggregate Intrinsic Value (in thousands) Balance, December 31, 2017 6,629,111 $ 14.15 8.2 $ 156,900 Options granted 2,163,700 $ 32.05 Options exercised and shares vested (952,231 ) $ 7.75 Options cancelled (497,273 ) $ 20.34 Balance, September 30, 2018 7,343,307 $ 19.83 8.0 $ 54,674 Options vested and expected to vest as of September 30, 2018 7,061,816 $ 19.55 7.9 $ 54,570 Options exercisable as of September 30, 2018 3,421,849 $ 12.86 8.0 $ 49,338 The aggregate intrinsic values of options outstanding, exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the market price for shares of our common stock as of September 30, 2018. The 2013 Plan provided for early exercise, therefore, all our outstanding stock options issued under that plan are exercisable. As of September 30, 2018 and 2017, there was $59.0 million and $44.3 million, respectively, of unrecognized stock-based compensation expense related to stock options, which is expected to be recognized over the weighted-average remaining vesting period of 2.6 years and 2.7 years, respectively. Restricted stock unit, or RSU, activity under the 2015 Plan is set forth below: Shares Weighted Average Grant Date Fair Value Unvested Balance, December 31, 2017 16,638 $ 35.41 Awarded 346,962 33.44 Released (4,160 ) 39.55 Forfeited (20,189 ) 34.07 Unvested Balance, September 30, 2018 339,251 $ 33.63 RSUs are measured based on the fair market value of the underlying stock on the date of grant and recognized as expense on a straight-line basis over the employee’s requisite service period (generally the vesting period). As of September 30, 2018 In connection with the expansion and extension of our long-term exclusive commercial supply agreement with GPC, we issued 300,000 shares of restricted common stock in January 2018 (see Note 5). The restricted common stock vests in four tranches over a 3.5 year period and is measured based on the fair market value of the underlying stock as the shares vest. As of September 30, 2018, 75,000 shares had vested, and the remaining shares were restricted. As of September 30, 2018, total estimated unrecognized expense related to these restricted shares was $5.7 million based upon the fair market value of our common stock on September 30, 2018, which is expected to be recognized over the remaining vesting period of 2.8 years as general and administrative expense. Stock-based compensation expense recognized during the quarter and nine months ended September 30, 2018 related to these shares was $0.7 million and $2.6 million, respectively. Valuation Assumptions The weighted-average assumptions used to estimate the fair value of stock options using the Black-Scholes option valuation model and the resulting weighted average fair value of stock options granted were as follows : Quarter Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) 6.1 6.0 6.0 6.0 Expected volatility 66.5 % 71.1 % 68.2 % 73.3 % Risk free interest rate 2.8 % 2.1 % 2.4 % 2.1 % Dividend yield — % — % — % — % Weighted average estimated fair value $ 17.19 $ 14.57 $ 20.09 $ 12.89 Stock-Based Compensation Expense Stock-based compensation expense, net of estimated forfeitures, reflected in the condensed consolidated statements of comprehensive loss is as follows (in thousands): Quarter Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 2,406 $ 1,305 $ 7,376 $ 3,482 General and administrative 5,976 2,774 17,287 8,389 Total stock-based compensation expense $ 8,382 $ 4,079 $ 24,663 $ 11,871 During the quarter and nine months ended September 30, 2018, we recorded approximately $0.7 million and $2.8 million of stock-based compensation expense related to the acceleration of certain former executives’ stock options.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share	7. Net Loss per Share Basic net loss per share is calculated based on the weighted-average number of common shares outstanding during the periods presented. For periods in which we have generated a net loss, basic and diluted net loss per share are the same due to the requirement to exclude potentially dilutive securities, consisting of common shares underlying outstanding stock options and restricted stock units, which would have an anti-dilutive effect on net loss per share. The following common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because their inclusion would have been antidilutive: Quarter Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Stock options 7,343,307 6,930,551 7,343,307 6,930,551 Restricted stock units 339,251 — 339,251 —

Related Party Transaction

Related Party Transaction	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transaction	8. Related Party Transaction In June 2017, Mark McDade, a member of our Board of Directors, joined the Board of Directors of MyHealthTeams, a private company that creates social networks for people living with chronic conditions by partnering with pharmaceutical and healthcare companies. We entered into an agreement with MyHealthTeams in 2015 under which they provide services to us. During the quarters ended September 30, 2018 and 2017, there were payments of $0.1 million in each period to MyHealthTeams pursuant to such agreement. During the nine months ended September 30, 2018 and 2017, there were payments of $0.1 million in each period to MyHealthTeams pursuant to such agreement. At September 30, 2018 and December 31, 2017, t here were no accrued liabilities due under the MyHealthTeams agreement In February and March 2018, we issued and sold an aggregate of 6,325,000 shares of our common stock in an underwritten public offering at a price to the public of $32.00 per share for total net proceeds of $190.4 million. The following aggregate number of shares of common stock were sold to our owners of more than 5% of our common stock, directors, or executive officers during the underwritten public offering: Number of Shares of Common Stock (#) Aggregate Purchase Price ($) Owners of More Than 5% of Our Common Stock Nestlé Health Science US Holdings, Inc. 937,500 30,000,000 Board of Directors Patrick G. Enright 15,593 498,976 Kathryn E. Falberg 30,000 960,000 Mark T. Iwicki 9,375 300,000 Officers Eric H. Bjerkholt 3,125 100,000

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Preparation	Basis of Preparation The accompanying condensed consolidated financial statements have been prepared in accordance with Generally Accepted Accounting Principles, or GAAP, in the United States and applicable rules and regulations of the Securities and Exchange Commission, or SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2017, has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These condensed consolidated financial statements have been prepared on the same basis as our annual financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of our financial information. The results of operations for the quarter and nine months ended September 30, 2018, are not necessarily indicative of the results to be expected for the year ending December 31, 2018, or for any other interim period or for any other future year. We operate in one reportable segment. The accompanying condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2017, included in our Annual Report on Form 10-K filed with the SEC.
Basis of Consolidation	Basis of Consolidation The accompanying condensed consolidated financial statements include the accounts of our wholly-owned subsidiaries. All significant intercompany transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of the accompanying condensed consolidated financial statements in accordance with U.S. GAAP requires us 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 condensed consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions.
Significant Accounting Policies	Significant Accounting Policies There have been no significant changes to the accounting policies during the quarter and nine months ended September 30, 2018, as compared to the significant accounting policies described in Note 2 of the “Notes to Consolidated Financial Statements” in our audited consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2017, except as noted below.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In May 2017, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting We adopted ASU 2017-09 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows – Restricted Cash (Topic 230) We adopted ASU 2016-18 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. In October 2016, the FASB, issued ASU, 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers Other Than Inventory We adopted ASU 2016-16 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments We adopted ASU 2016-15 in the first quarter of 2018. There was no material impact to our condensed consolidated financial statements as a result of adopting this standard.
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued Accounting Standards Update No. 2018-13, Fair Value Measurement: Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In March 2017, the FASB issued ASU 2017-08, Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities We are currently evaluating the impact that the adoption of ASU 2017-08 will have on our consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU 2016-13, Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In February 2016, the FASB, issued ASU, No. 2016-02, Leases (Topic 842) Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements
Fair Value Measurements	We define fair value 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 Our valuation techniques are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect our market assumptions. We classify these inputs into the following hierarchy: • Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; • Level 2—Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and • Level 3—Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.

Available-for-Sale Securities_2

Available-for-Sale Securities and Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Financial Instruments Measured at Fair Value on a Recurring Basis	The following tables set forth our financial instrumen ts that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 75,921 $ — $ — $ 75,921 U.S. government securities — 2,499 — $ 2,499 Total cash and cash equivalents $ 75,921 $ 2,499 $ — $ 78,420 Investments: Agency securities $ — $ 16,899 $ — $ 16,899 Corporate securities — 49,099 — 49,099 Commercial paper — 3,999 — 3,999 U.S. government securities — 106,832 — 106,832 Total investments $ — $ 176,829 $ — $ 176,829 December 31, 2017 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 39,072 $ — $ — $ 39,072 Corporate securities — 999 — 999 Commercial paper — 33,416 — 33,416 Total cash and cash equivalents $ 39,072 $ 34,415 $ — $ 73,487 Investments: Agency securities — 12,718 — 12,718 Corporate securities — 28,345 — 28,345 Commercial paper — 21,432 — 21,432 U.S. government securities — 46,448 — 46,448 Total investments $ — $ 108,943 $ — $ 108,943
Summary of Aggregate Market Value, Cost Basis and Gross Unrealized Gains and Losses of Available-for-Sale Investments by Security Type	The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by security type, classified in cash equivalents and investments, as of September 30, 2018 and December 31, 2017, are as follows (in thousands): September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Total Fair Value Agency securities $ 16,906 $ — $ (7 ) $ 16,899 Corporate securities 49,137 1 (39 ) 49,099 Commercial paper 3,999 — — 3,999 U.S. government securities 109,424 — (93 ) 109,331 Total available-for-sale investments $ 179,466 $ 1 $ (139 ) $ 179,328 December 31, 2017 Amortized Cost Gross unrealized gains Gross unrealized losses Total fair value Agency securities $ 12,729 $ — $ (11 ) $ 12,718 Corporate securities 29,369 1 (26 ) 29,344 Commercial paper 54,848 — — 54,848 U.S. government securities 46,520 — (72 ) 46,448 Total available-for-sale investments $ 143,466 $ 1 $ (109 ) $ 143,358

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Summary of Property And Equipment, Net	Property and equipment, net consists of the following (in thousands): September 30, 2018 December 31, 2017 Furniture and equipment $ 2,052 $ 1,655 Computer equipment 1,783 1,410 Manufacturing equipment 885 830 Leased equipment 100 100 Leasehold improvements 2,685 2,685 Buildings 688 688 Construction in progress 19,626 11,490 Property and equipment, gross 27,819 18,858 Less: accumulated depreciation (2,774 ) (1,653 ) Property and equipment, net $ 25,045 $ 17,205
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): September 30, 2018 December 31, 2017 Compensation and benefits $ 7,294 $ 6,205 Research and development 13,203 12,716 Professional and consulting 2,601 2,370 Other 447 187 Total accrued liabilities $ 23,545 $ 21,478

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	Option activity under the 2015 Plan and 2013 Plan is set forth below: Options Outstanding Number Options and Unvested Shares Weighted- Average Exercise Price Weighted Average Remaining Contractual Life (in years) Aggregate Intrinsic Value (in thousands) Balance, December 31, 2017 6,629,111 $ 14.15 8.2 $ 156,900 Options granted 2,163,700 $ 32.05 Options exercised and shares vested (952,231 ) $ 7.75 Options cancelled (497,273 ) $ 20.34 Balance, September 30, 2018 7,343,307 $ 19.83 8.0 $ 54,674 Options vested and expected to vest as of September 30, 2018 7,061,816 $ 19.55 7.9 $ 54,570 Options exercisable as of September 30, 2018 3,421,849 $ 12.86 8.0 $ 49,338
Restricted Stock Unit Activity	Restricted stock unit, or RSU, activity under the 2015 Plan is set forth below: Shares Weighted Average Grant Date Fair Value Unvested Balance, December 31, 2017 16,638 $ 35.41 Awarded 346,962 33.44 Released (4,160 ) 39.55 Forfeited (20,189 ) 34.07 Unvested Balance, September 30, 2018 339,251 $ 33.63
Schedule of Weighted Average Assumptions to Fair Value of Stock Options	The weighted-average assumptions used to estimate the fair value of stock options using the Black-Scholes option valuation model and the resulting weighted average fair value of stock options granted were as follows : Quarter Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Expected term (in years) 6.1 6.0 6.0 6.0 Expected volatility 66.5 % 71.1 % 68.2 % 73.3 % Risk free interest rate 2.8 % 2.1 % 2.4 % 2.1 % Dividend yield — % — % — % — % Weighted average estimated fair value $ 17.19 $ 14.57 $ 20.09 $ 12.89
Summary of Stock-based Compensation Expense Net of Estimated Forfeitures	Stock-Based Compensation Expense Stock-based compensation expense, net of estimated forfeitures, reflected in the condensed consolidated statements of comprehensive loss is as follows (in thousands): Quarter Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Research and development $ 2,406 $ 1,305 $ 7,376 $ 3,482 General and administrative 5,976 2,774 17,287 8,389 Total stock-based compensation expense $ 8,382 $ 4,079 $ 24,663 $ 11,871

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss per Share	The following common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because their inclusion would have been antidilutive: Quarter Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Stock options 7,343,307 6,930,551 7,343,307 6,930,551 Restricted stock units 339,251 — 339,251 —

Related Party Transaction (Tabl

Related Party Transaction (Tables)	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Schedule of Aggregate Number of Common Stock Sold to Owners	The following aggregate number of shares of common stock were sold to our owners of more than 5% of our common stock, directors, or executive officers during the underwritten public offering: Number of Shares of Common Stock (#) Aggregate Purchase Price ($) Owners of More Than 5% of Our Common Stock Nestlé Health Science US Holdings, Inc. 937,500 30,000,000 Board of Directors Patrick G. Enright 15,593 498,976 Kathryn E. Falberg 30,000 960,000 Mark T. Iwicki 9,375 300,000 Officers Eric H. Bjerkholt 3,125 100,000

Formation and Business of the_2

Formation and Business of the Company - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ 51,702	$ 31,792	$ 153,754	$ 90,159
Net cash used in operating activities			117,578	$ 67,678
Accumulated deficit	419,236		419,236		$ 265,482
Cash, cash equivalents and investments	$ 255,200		$ 255,200

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018Segment
Accounting Policies [Abstract]
Number of reportable segments	1

Available-for-Sale Securities_3

Available-for-Sale Securities and Fair Value Measurements - Summary of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash and cash equivalents
Total cash and cash equivalents	$ 78,420	$ 73,487
Investments:
Total investments	176,829	108,943
Cash and Money Market Funds
Cash and cash equivalents
Total cash and cash equivalents	75,921	39,072
U.S. Government Securities
Cash and cash equivalents
Total cash and cash equivalents	2,499
Investments:
Total investments	106,832	46,448
Agency Securities
Investments:
Total investments	16,899	12,718
Corporate Debt Securities
Cash and cash equivalents
Total cash and cash equivalents		999
Investments:
Total investments	49,099	28,345
Commercial Paper
Cash and cash equivalents
Total cash and cash equivalents		33,416
Investments:
Total investments	3,999	21,432
Level 1
Cash and cash equivalents
Total cash and cash equivalents	75,921	39,072
Level 1 | Cash and Money Market Funds
Cash and cash equivalents
Total cash and cash equivalents	75,921	39,072
Level 2
Cash and cash equivalents
Total cash and cash equivalents	2,499	34,415
Investments:
Total investments	176,829	108,943
Level 2 | U.S. Government Securities
Cash and cash equivalents
Total cash and cash equivalents	2,499
Investments:
Total investments	106,832	46,448
Level 2 | Agency Securities
Investments:
Total investments	16,899	12,718
Level 2 | Corporate Debt Securities
Cash and cash equivalents
Total cash and cash equivalents		999
Investments:
Total investments	49,099	28,345
Level 2 | Commercial Paper
Cash and cash equivalents
Total cash and cash equivalents		33,416
Investments:
Total investments	$ 3,999	$ 21,432

Available-for-Sale Securities_4

Available-for-Sale Securities and Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair value assets transfers between level 1 to level 2	$ 0	$ 0	$ 0	$ 0
Fair value assets transfers between level 2 to level 1	0	0	$ 0	0
Available-for-sale securities, maturities period			12 months
Other than temporary impairment losses	$ 0	$ 0	$ 0	$ 0
Marketable securities unrealized losses position maximum period			12 months

Available-for-Sale Securities_5

Available-for-Sale Securities and Fair Value Measurements - Summary of Aggregate Market Value, Cost Basis, and Gross Unrealized Gains and Losses of Available for Sale Investments by Security Type (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	$ 179,466	$ 143,466
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(139)	(109)
Total Fair Value	179,328	143,358
Agency Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	16,906	12,729
Gross Unrealized Losses	(7)	(11)
Total Fair Value	16,899	12,718
Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	49,137	29,369
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(39)	(26)
Total Fair Value	49,099	29,344
Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	3,999	54,848
Total Fair Value	3,999	54,848
U.S. Government Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	109,424	46,520
Gross Unrealized Losses	(93)	(72)
Total Fair Value	$ 109,331	$ 46,448

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 27,819	$ 18,858
Less: accumulated depreciation	(2,774)	(1,653)
Property and equipment, net	25,045	17,205
Furniture and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,052	1,655
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,783	1,410
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	885	830
Leased Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	100	100
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,685	2,685
Buildings
Property Plant And Equipment [Line Items]
Property and equipment, gross	688	688
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 19,626	$ 11,490

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Compensation and benefits	$ 7,294	$ 6,205
Research and development	13,203	12,716
Professional and consulting	2,601	2,370
Other	447	187
Total accrued liabilities	$ 23,545	$ 21,478

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - Archer Daniels Midland Company $ in Millions	1 Months Ended	9 Months Ended
	Jan. 31, 2018USD ($)Trancheshares	Sep. 30, 2018USD ($)
Commitments And Contingencies [Line Items]
Long-term purchase commitment, restated agreement effective period after first delivery	10 years
Long-term purchase commitment, optional extension period	5 years
Long-term purchase commitment amount		$ 8.3
Long-term purchase commitment period		10 years
Restricted Stock | Common Stock
Commitments And Contingencies [Line Items]
Long-term purchase commitment, shares issued | shares	300,000
Long-term purchase commitment, shares issued, vesting period	3 years 6 months
Long-term purchase commitment, fair value	$ 11.7
Number of tranches for vesting of shares | Tranche	4

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested shares outstanding		7,343,307	7,343,307		6,629,111
Former Executive
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Accelerated stock compensation expense		$ 700,000	$ 2,800,000
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized employee stock-based compensation		59,000,000	$ 59,000,000	$ 44,300,000
Expected recognized over weighted-average remaining vesting period			2 years 7 months 6 days	2 years 8 months 12 days
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected recognized over weighted-average remaining vesting period			3 years 2 months 12 days
Unrecognized employee stock-based compensation		8,800,000	$ 8,800,000	$ 0
Restricted Stock | Common Stock | Archer Daniels Midland Company
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Long-term purchase commitment, shares issued	300,000
Long-term purchase commitment, shares issued, vesting period	3 years 6 months
Stock-based compensation, shares vested			75,000
Unrecognized employee stock-based compensation		5,700,000	$ 5,700,000
Expected recognized over weighted-average remaining vesting period			2 years 9 months 18 days
Stock-based compensation expense		$ 700,000	$ 2,600,000
2013 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		0	0
Unvested shares outstanding		0	0		46,973
Unvested shares issued		0	0		46,973
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		4,400,000	4,400,000
2013 and 2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested shares outstanding		7,300,000	7,300,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Number of Options and Unvested Shares
Number of Options and Unvested Shares, Beginning Balance	6,629,111
Number of Options and Unvested Shares, Options granted	2,163,700
Number of Options and Unvested Shares, Options exercised and shares vested	(952,231)
Number of Options and Unvested Shares, Options cancelled	(497,273)
Number of Options and Unvested Shares, Ending Balance	7,343,307	6,629,111
Number of Options and Unvested Shares, Options vested and expected to vest as of June 30, 2018	7,061,816
Number of Options and Unvested Shares, Options exercisable as of June 30, 2018	3,421,849
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 14.15
Weighted-Average Exercise Price, Options granted	32.05
Weighted-Average Exercise Price, Options exercised and shares vested	7.75
Weighted-Average Exercise Price, Options cancelled	20.34
Weighted-Average Exercise Price, Ending Balance	19.83	$ 14.15
Weighted-Average Exercise Price, Options vested and expected to vest as of June 30, 2018	19.55
Weighted-Average Exercise Price, Options exercisable as of June 30, 2018	$ 12.86
Weighted Average Remaining Contractual Life (in years)
Weighted Average Remaining Contractual Life (in years), Balance	8 years	8 years 2 months 12 days
Weighted Average Remaining Contractual Life, Options vested and expected to vest as of June 30, 2018	7 years 10 months 24 days
Weighted Average Remaining Contractual Life, Options exercisable as of June 30, 2018	8 years
Aggregate Intrinsic Value, Balance	$ 54,674	$ 156,900
Aggregate Intrinsic Value, Options vested and expected to vest as of June 30, 2018	54,570
Aggregate Intrinsic Value, Options exercisable as of June 30, 2018	$ 49,338

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock Unit Activity (Details) - 2015 Plan - Restricted Stock Units (RSUs) [Member]	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested Shares, Beginning Balance | shares	16,638
Shares, Awarded | shares	346,962
Shares, Released | shares	(4,160)
Shares, Forfeited | shares	(20,189)
Unvested Shares, Ending Balance | shares	339,251
Unvested Weighted Average Grant Date Fair Value, Beginning Balance | $ / shares	$ 35.41
Weighted Average Grant Date Fair Value, Awarded | $ / shares	33.44
Weighted Average Grant Date Fair Value, Released | $ / shares	39.55
Weighted Average Grant Date Fair Value, Forfeited | $ / shares	34.07
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 33.63

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Weighted Average Assumptions to Fair Value of Stock Options (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Methodology [Abstract]
Expected term (in years)	6 years 1 month 6 days	6 years	6 years	6 years
Expected volatility	66.50%	71.10%	68.20%	73.30%
Risk free interest rate	2.80%	2.10%	2.40%	2.10%
Weighted average estimated fair value	$ 17.19	$ 14.57	$ 20.09	$ 12.89

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-based Compensation Expense Net of Estimated Forfeitures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 8,382	$ 4,079	$ 24,663	$ 11,871
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	2,406	1,305	7,376	3,482
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 5,976	$ 2,774	$ 17,287	$ 8,389

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	7,343,307	6,930,551	7,343,307	6,930,551
Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	339,251		339,251

Related Party Transaction - Add

Related Party Transaction - Additional Information (Details) - USD ($)	2 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Aggregate number of common stock issued and sold	6,325,000
Underwritten public offering price per share	$ 32
Proceeds from issuance of common stock	$ 190,400,000
MyHealth Teams
Related Party Transaction [Line Items]
Payments to service agreement		$ 100,000	$ 100,000	$ 100,000	$ 100,000
Accrued liabilities due		$ 0		$ 0		$ 0

Related Party Transactions - Sc

Related Party Transactions - Schedule of Aggregate Number of Common Stock Sold to Owners (Details) - USD ($)	2 Months Ended	9 Months Ended
	Mar. 31, 2018	Sep. 30, 2018
Related Party Transaction [Line Items]
Number of Shares of Common Stock	6,325,000
Aggregate Purchase Price	$ 190,400,000
Nestlé Health Science US Holdings, Inc.
Related Party Transaction [Line Items]
Number of Shares of Common Stock		937,500
Aggregate Purchase Price		$ 30,000,000
Patrick G. Enright
Related Party Transaction [Line Items]
Number of Shares of Common Stock		15,593
Aggregate Purchase Price		$ 498,976
Kathryn E. Falberg
Related Party Transaction [Line Items]
Number of Shares of Common Stock		30,000
Aggregate Purchase Price		$ 960,000
Mark T. Iwicki
Related Party Transaction [Line Items]
Number of Shares of Common Stock		9,375
Aggregate Purchase Price		$ 300,000
Eric H. Bjerkholt
Related Party Transaction [Line Items]
Number of Shares of Common Stock		3,125
Aggregate Purchase Price		$ 100,000

Related Party Transactions - _2

Related Party Transactions - Schedule of Aggregate Number of Common Stock Sold to Owners (Parenthetical) (Details)	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Aggregate number of shares of common stock sold, ownership percentage of common stock	Owners of More Than 5%