Aimmune Therapeutics (AIMT) 10-K 28 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	AIMT
Entity Registrant Name	AIMMUNE THERAPEUTICS, INC.
Entity Central Index Key	1,631,650
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		62,186,194
Entity Public Float			$ 1,163,912,244

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 107,511	$ 73,487
Short-term investments	196,421	108,943
Prepaid expenses and other current assets	8,687	6,681
Total current assets	312,619	189,111
Property and equipment, net	26,328	17,205
Prepaid expenses and other assets	608	618
Total assets	339,555	206,934
Current liabilities:
Accounts payable	8,833	5,095
Accrued liabilities	29,144	21,478
Other current liabilities	35	26
Total current liabilities	38,012	26,599
Other liabilities	2,596	2,530
Total liabilities	40,608	29,129
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, par value $0.0001 per share - 10,000 shares authorized at December 31, 2018 and 2017; 0 shares issued and outstanding at December 31, 2018 and 2017
Common stock, par value $0.0001 per share—290,000 shares authorized as of December 31, 2018 and 2017; 62,142 and 51,091 shares issued and outstanding as of December 31, 2018 and 2017, respectively (including 0 and 47 shares subject to repurchase, legally issued and outstanding as of December 31, 2018 and 2017, respectively)	6	5
Additional paid-in capital	775,283	443,390
Accumulated other comprehensive loss	(108)	(108)
Accumulated deficit	(476,234)	(265,482)
Total stockholders’ equity	298,947	177,805
Total liabilities and stockholders’ equity	$ 339,555	$ 206,934

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Preferred stock, par value	$ 0.0001	$ 0.0001
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	$ 0.0001	$ 0.0001
Common stock, shares authorized	290,000,000	290,000,000
Common stock, shares issued	62,142,000	51,091,000
Common stock, shares outstanding	62,142,000	51,091,000
Common Stock Subject to Repurchase
Common stock, shares issued	0	47,000
Common stock, shares outstanding	0	47,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses
Research and development	$ 133,420	$ 89,325	$ 54,642
General and administrative	81,921	43,949	26,885
Total operating expenses	215,341	133,274	81,527
Loss from operations	(215,341)	(133,274)	(81,527)
Interest income, net	4,650	2,005	703
Loss before provision for income taxes	(210,691)	(131,269)	(80,824)
Provision for income taxes	61	56
Net loss	(210,752)	(131,325)	(80,824)
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on investments		(81)	61
Comprehensive loss	$ (210,752)	$ (131,406)	$ (80,763)
Net loss per share, basic and diluted	$ (3.67)	$ (2.61)	$ (1.89)
Weighted average shares used in computing net loss per share, basic and diluted	57,403	50,401	42,751

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated deficit
Beginning balance at Dec. 31, 2015	$ 206,251	$ 4	$ 259,668	$ (88)	$ (53,333)
Beginning balances (in shares) at Dec. 31, 2015		42,239
Issuance of common stock upon exercise of vested options	2,843		2,843
Issuance of common stock upon exercise of vested options (in shares)		413
Issuance of common stock upon securities purchase agreement	145,000	$ 1	144,999
Issuance of common stock upon securities purchase agreement (in shares)		7,552
Stock-based compensation	12,641		12,641
Other comprehensive gain (loss)	61			61
Net loss	(80,824)				(80,824)
Ending balances at Dec. 31, 2016	285,972	$ 5	420,151	(27)	(134,157)
Ending balances (in shares) at Dec. 31, 2016		50,204
Issuance of common stock upon exercise of vested options and vesting of restricted stock units	6,520		6,520
Issuance of common stock upon exercise of vested options and vesting of restricted stock units (in shares)		887
Stock-based compensation	16,719		16,719
Other comprehensive gain (loss)	(81)			(81)
Net loss	(131,325)				(131,325)
Ending balances at Dec. 31, 2017	177,805	$ 5	443,390	(108)	(265,482)
Ending balances (in shares) at Dec. 31, 2017		51,091
Issuance of common stock upon exercise of vested options and vesting of restricted stock units	10,726	$ 0	10,726
Issuance of common stock upon exercise of vested options and vesting of restricted stock units (in shares)		1,188
Issuance of common stock upon public offering	190,436	$ 1	190,435
Issuance common stock upon initial public offering (in shares)		6,325
Issuance of common stock upon securities purchase agreement	98,000		98,000
Issuance of common stock upon securities purchase agreement (in shares)		3,538
Stock-based compensation	32,732		32,732
Net loss	(210,752)				(210,752)
Ending balances at Dec. 31, 2018	$ 298,947	$ 6	$ 775,283	$ (108)	$ (476,234)
Ending balances (in shares) at Dec. 31, 2018		62,142

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (210,752)	$ (131,325)	$ (80,824)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation expense	1,598	966	534
Stock-based compensation expense	32,732	16,719	12,641
Investment premium amortization, net	(886)	511	919
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,995)	(1,435)	3,967
Accounts payable	2,688	2,315	(994)
Accrued liabilities	7,412	11,557	6,803
Other liabilities	75	1,087	340
Net cash used in operating activities	(169,128)	(99,605)	(56,614)
Cash flows from investing activities:
Purchases of property and equipment	(9,420)	(6,425)	(7,665)
Purchases of investments	(261,293)	(129,534)	(197,178)
Maturities of investments	174,703	178,521	160,947
Net cash provided by (used in) investing activities	(96,010)	42,562	(43,896)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	288,435	6,520	145,000
Net cash proceeds from exercise of stock options, including early exercise	10,727		2,843
Net cash provided by financing activities	299,162	6,520	147,843
Net increase (decrease) in cash and cash equivalents	34,024	(50,523)	47,333
Cash and cash equivalents at the beginning of the year	73,487	124,010	76,677
Cash and cash equivalents at the end of the year	107,511	73,487	124,010
Supplemental schedule of non-cash investing and financing activities:
Purchases of property and equipment reported in accounts payable	1,301	1,355	$ 558
Supplemental cash flow disclosures:
Cash paid for taxes	$ 148	$ 39

Formation and Business of the C

Formation and Business of the Company	12 Months Ended
	Dec. 31, 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 year ended December 31, 2018, we incurred a net loss of $210.8 million and used $169.1 million of cash in operations. As of December 31, 2018, we had an accumulated deficit of $476.2 million and we do not expect to experience positive cash flows in the near future. As of December 31, 2018, we had cash, cash equivalents and investments of $303.9 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 of AR101, our lead CODIT TM

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Preparation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States, or U.S. GAAP, and include the accounts of our wholly-owned subsidiaries. All significant intercompany transactions have been eliminated. We operate in one reportable segment in the United States. Foreign Currency Translation Our functional currency and the functional currency of all of our subsidiaries is the United States dollar. Accordingly, monetary assets and liabilities in the non-functional currency of these subsidiaries are remeasured using exchange rates in effect at the end of the period. Costs in local currency are remeasured using average exchange rates for the period, except for costs related to those balance sheet items that are remeasured using historical exchange rates. The resulting remeasurement gains and losses are included in the consolidated statements of operations and comprehensive loss as incurred and have not been material for all periods presented. Use of Estimates The preparation of the accompanying 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 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. Cash and Cash Equivalents We consider all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents, which are carried at estimated fair value, consist primarily of money market funds and certain available-for-sale investments with maturities of three months or less. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker in making decisions regarding resource allocation and assessing performance. We have one operating segment. Concentration of Credit Risk Financial instruments that potentially subject us to a concentration of credit risk consist of cash and cash equivalents and certain investments in money market funds, agency securities, corporate securities, U.S. government securities and commercial paper. Bank deposits are primarily held by a single financial institution and these deposits may exceed insured limits. We are exposed to credit risk in the event of default by the financial institution holding our cash and cash equivalents and issuers of investments that are recorded on our consolidated balance sheets. We mitigate our risk by investing in high-grade instruments and limiting the concentration in any one issuer, which limits our exposure. Investments Our investments consist of available-for-sale securities. Investments with original maturities of greater than 90 days but less than one (1) year are classified as short-term on the consolidated balance sheets. Investments with original maturities greater than one (1) year are classified as long-term on the consolidated balance sheets. Our investments in available-for-sale securities are reported at estimated fair value. Available-for-sale securities consist primarily of agency securities, corporate securities, U.S. government securities and commercial paper. Unrealized gains and losses related to changes in the fair value of securities are recognized in accumulated other comprehensive loss, net of tax, on our consolidated balance sheets. Changes in the fair value of available-for-sale securities impact the consolidated statements of operations and comprehensive loss only when such securities are sold or an other-than-temporary impairment is recognized. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. We regularly review our investment portfolio to determine if any security is other-than-temporarily impaired, which would require us to record an impairment charge in the period any such determination is made. We consider factors such as the duration, severity and the reason for the decline in value, the financial condition of the issuer and any changes thereto, 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. Our assessment on whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security. Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the respective assets. Maintenance and repairs are charged to the consolidated statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss, if any, is reflected in the consolidated statements of operations and comprehensive loss. The useful lives of property and equipment are as follows: Computer equipment 3 years Furniture and office equipment 4 years Manufacturing equipment 7 years Buildings 25 years Leasehold improvements Shorter of remaining lease terms or useful life Impairment of Long-Lived Assets We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows expected to result from the use of the asset and its eventual disposition. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired assets. We have not recorded impairment of any long-lived assets in the periods presented. Leases Leases related to our corporate headquarters are classified as operating leases. Rent expense is recognized on a straight-line basis over the terms of the leases and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Incentives granted under facilities leases are deferred and recognized as adjustments to rental expense on a straight-line basis over the term of the lease. In June 2015, we signed a lease for a manufacturing facility in Clearwater, Florida. We were considered the deemed owner for accounting purposes. See Note 5, “Commitments and Contingencies,” for further details. Research and Development We expense research and development costs as incurred. We record accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of pre-clinical studies and clinical trials and contract manufacturing activities. These costs are a significant component of our research and development expenses. We accrue for these costs based on factors such as estimates of the work completed and in accordance with agreements established with our third-party service providers under the service agreements. We make significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, we adjust our accrued liabilities. We have not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed, number of patients enrolled and the rate of patient enrollments may vary from our estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to our accruals could materially affect our results of operations. Stock-Based Compensation We recognize compensation costs related to stock options granted to employees and directors based on the estimated fair value of the awards on the date of grant, net of estimated forfeitures. We estimate the grant date fair value using the Black-Scholes option-pricing model. The grant date fair value of the stock-based awards is generally recognized on a straight-line basis over the requisite service period, which is generally the vesting period of the respective awards. The fair value of the shares granted pursuant to the 2015 Employee Stock Purchase Plan (the “2015 ESPP”) In determining the fair value of the stock-based awards used to calculate stock-based compensation expense, we use the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires judgment to determine. • Expected Term. The expected term of stock-based awards represents the weighted average period the stock-based awards are expected to be outstanding. We have opted to use the simplified method for estimating the expected term as provided by the Securities and Exchange Commission Staff Accounting Bulletin, or SAB, 110 as our stock-based awards are considered “plain vanilla”. The simplified method calculates the expected term as the average time-to-vesting and the contractual life of the options. We plan to continue to use the simplified method under SAB 110 until we have sufficient exercise history as a publicly traded company. • Expected Volatility. As we have limited trading history for our common stock, the expected stock price volatility assumption is determined based on the historical volatilities of a group of industry peers as well as the historical volatility of our own common stock since we began trading subsequent to our IPO in August 2015. Industry peers consist of several public companies in the biopharmaceutical industry with comparable characteristics including enterprise value, risk profiles and position within the industry. We intend to continue to consistently apply this process using the same or similar public companies until a sufficient amount of historical information regarding the volatility of our own common stock share price becomes available, or unless circumstances change such that the identified companies are no longer similar to us, in which case, more suitable companies whose share prices are publicly available would be utilized in the calculation. • Risk-Free Interest Rate. The risk-free interest rate is based on the implied yield available on U.S. Treasury zero-coupon issues in effect at the time of grant for periods corresponding with the expected term of the stock-based award. • Expected Dividend Yield. We have never paid dividends on our common stock and have no plans to pay dividends on our common stock. Therefore, we used an expected dividend yield of zero for all years presented Restricted Stock Units, or “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). In addition to the Black-Scholes assumptions, we estimate our forfeiture rate based on an analysis of our actual forfeitures and will continue to evaluate the adequacy of the forfeiture rate based on actual forfeiture experience, analysis of employee turnover behavior and other factors. The impact from any forfeiture rate adjustment would be recognized in full in the period of adjustment, and if the actual number of future forfeitures differs from our estimates, we might be required to record adjustments to stock-based compensation in future periods. We will continue to use judgment in evaluating the expected volatility, expected terms and forfeiture rates utilized for our stock-based compensation expense calculations on a prospective basis. Income Taxes We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of reported assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. Tax benefits from uncertain tax positions are recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on technical merits. The amount recognized is measured as the largest amount of tax benefit that is greater than 50% likely of being realized upon effective settlement. Comprehensive Income or Loss Comprehensive income or loss is defined as the change in equity during a period from transactions and other events, excluding changes resulting from investments from owners and distributions to owners. Other comprehensive loss includes net loss and unrealized gains and losses on available-for-sale investments. 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: Year Ended December 31, 2018 2017 2016 Stock options 7,133,113 6,629,111 5,429,267 Restricted stock units 309,847 16,638 17,000 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 orderly transaction between market participants on the measurement date. 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. Financial instruments include cash equivalents, investments, accounts payable, and accrued liabilities. Our cash equivalents and investments are carried at estimated fair value and remeasured on a recurring basis. The carrying value of accounts payable and accrued liabilities approximate their estimated fair value due to the relatively short-term nature of these instruments. 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. In accordance with fair value accounting requirements, companies may choose to measure eligible financial instruments and certain other items at fair value. We have not elected the fair value option for any eligible financial instruments. 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 impact to our 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 impact to our 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 impact to our 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 impact to our consolidated financial statements as a result of adopting this standard. Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement: Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In June 2016, the FASB issued ASU 2016-13, Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments after December 15, 2019 and interim periods within those fiscal years. Early adoption is permitted. We are currently evaluating the impact that the adoption of ASU 2016-13 will have on our consolidated financial statements and related disclosures. In February 2016, the FASB, issued ASU 2016-02, Leases (Topic 842) We will adopt the standard effective in the first quarter of 2019 and will not restate comparative periods upon adoption. We will elect a package of practical expedients for leases that commenced prior to January 1, 2019 and will not reassess: (i) whether any expired or existing contracts are or contain leases; (ii) lease classification for any expired or existing leases; and (iii) initial direct costs capitalization for any existing leases. We will make an accounting policy election to keep leases with an initial term of 12 months or less off of the balance sheet. We will recognize those lease payments in the consolidated statements of operations on a straight-line basis over the lease term. We currently expect that our operating lease commitments will be subject to the new standard and recognized as operating lease liabilities and right-of-use assets upon our adoption of Topic 842, which will increase the total assets and total liabilities that we report relative to such amounts prior to adoption. We estimate adoption of the standard will result in the recognition of right of use assets and additional net lease liabilities of approximately $12.0 million, respectively. Refer to Note 5 for further information on our operating lease commitments. We plan to adopt Topic 842 using the alternative modified retrospective approach with the cumulative effect of adoption recognized to retained earnings on January 1, 2019. We do not believe the new standard will have a material impact on our consolidated statements of operations, nor will it have a notable impact on our liquidity. The standard will also have no impact on our debt-covenant compliance under our current agreements.

Available-for-Sale Securities a

Available-for-Sale Securities and Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Available-for-Sale Securities and Fair Value Measurements	3. Available-for-Sale Securities and Fair Value Measurements The following table sets forth our financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): December 31, 2018 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 107,511 $ — $ — $ 107,511 Total cash and cash equivalents $ 107,511 $ — $ — $ 107,511 Investments: Agency securities — 17,352 — 17,352 Corporate securities — 54,474 — 54,474 Commercial paper — 5,965 — 5,965 US government securities — 118,630 — 118,630 Total investments $ — $ 196,421 $ — $ 196,421 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 US 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 years ended December 31, 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 December 31, 2018 and December 31, 2017 are as are as follows (in thousands): December 31, 2018 Amortized Cost Gross unrealized gains Gross unrealized losses Total fair value Agency securities $ 17,361 $ - $ (9 ) $ 17,352 Corporate securities 54,536 — (62 ) 54,474 Commercial paper 5,965 — — 5,965 US government securities 118,667 14 (51 ) 118,630 Total available-for-sale investments $ 196,529 $ 14 $ (122 ) $ 196,421 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 US government securities 46,520 — (72 ) 46,448 Total available-for-sale investments $ 143,466 $ 1 $ (109 ) $ 143,358 At December 31, 2018, all of the available-for-sale securities have contractual maturities within one year. 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 years ended December 31, 2018, 2017, and 2016 we did not recognize any other-than-temporary impairment losses. All marketable securities with unrealized losses have been in a loss position for less than twelve months. 3. Available-for-Sale Securities and Fair Value Measurements The following table sets forth our financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): December 31, 2018 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 107,511 $ — $ — $ 107,511 Total cash and cash equivalents $ 107,511 $ — $ — $ 107,511 Investments: Agency securities — 17,352 — 17,352 Corporate securities — 54,474 — 54,474 Commercial paper — 5,965 — 5,965 US government securities — 118,630 — 118,630 Total investments $ — $ 196,421 $ — $ 196,421 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 US 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 years ended December 31, 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 December 31, 2018 and December 31, 2017 are as are as follows (in thousands): December 31, 2018 Amortized Cost Gross unrealized gains Gross unrealized losses Total fair value Agency securities $ 17,361 $ - $ (9 ) $ 17,352 Corporate securities 54,536 — (62 ) 54,474 Commercial paper 5,965 — — 5,965 US government securities 118,667 14 (51 ) 118,630 Total available-for-sale investments $ 196,529 $ 14 $ (122 ) $ 196,421 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 US government securities 46,520 — (72 ) 46,448 Total available-for-sale investments $ 143,466 $ 1 $ (109 ) $ 143,358 At December 31, 2018, all of the available-for-sale securities have contractual maturities within one year. 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 years ended December 31, 2018, 2017, and 2016 we did not recognize any other-than-temporary impairment losses. All marketable securities with unrealized losses have been in a loss position for less than twelve months.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 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): Year Ended December 31, 2018 2017 Furniture and office equipment $ 2,221 $ 1,655 Computer equipment 2,073 1,410 Manufacturing equipment 1,733 830 Leased equipment 100 100 Leasehold improvements 4,469 2,685 Buildings 688 688 Construction in progress 18,295 11,490 Property and equipment, gross 29,579 18,858 Less: accumulated depreciation (3,251 ) (1,653 ) Property and equipment, net $ 26,328 $ 17,205 Depreciation expense for the years ended December 31, 2018, 2017 and 2016 was $1.6 million, $1.0 million and $0.5 million, respectively. Accrued Liabilities Accrued liabilities consist of the following (in thousands): Year Ended December 31, 2018 2017 Compensation and benefits $ 8,912 $ 6,205 Research and development 15,504 12,716 Professional and consulting 4,691 2,370 Other 37 187 Total accrued liabilities $ 29,144 $ 21,478

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. Commitments and Contingencies Facility Leases In 2015, we entered into lease agreements for our corporate headquarters in Brisbane, California for 38,020 square feet of office space. In June 2017, we entered into a second lease amendment, under which we leased an additional 14,841 square feet of office space. The term for the additional space commenced on January 1, 2018 and terminates on June 30, 2024. Additionally, the term of the existing office space has been extended so that it is coterminous with the new space.We are responsible for operating expenses over base operating expenses as defined in the original lease agreement. In June 2015, we signed a facility lease for a manufacturing facility for approximately 20,000 square feet of manufacturing space in Clearwater, Florida. The initial term of the lease is for 120 months. For accounting purposes, due to the nature and extent of our involvement with the construction of this manufacturing facility, we were considered to be the owner of the assets during the construction period through the lease commencement date, even though the lessor is responsible for funding and repairing components of the building shell and constructing a portion of the related building infrastructure. We capitalized $0.7 million for costs incurred by the lessor in constructing the building shell and recognized a corresponding amount included within other liabilities. Construction to this building commenced in July 2015 and as of December 31, 2018, we have incurred approximately $24.5 million of construction and equipment costs related to the building, of which $17.8 million is recorded in construction in progress. We are responsible for operating expenses including real estate taxes as defined in the manufacturing facility lease agreement. In September 2018, we entered into a lease for office space in Durham, North Carolina for 5,099 square feet of office space. The lease is expected to expire in February 2024. In November 2018, we entered into a lease for part of a floor of office space in London, United Kingdom. The lease is expected to expire in November 2023. As a part of the lease, we have a contractual requirement to remove the tenant improvements and restore the leased office space to a condition as specified in the lease agreement. Total future aggregate minimum lease payments under our operating leases, are as follows (in thousands): Year Ended December 31, 2019 $ 3,133 2020 3,330 2021 3,413 2022 3,497 2023 3,378 and thereafter 1,744 Total $ 18,495 Rent expense under operating leases for the years ended December 31, 2018, 2017 and 2016 was $3.8 million, $2.4 million and $1.7 million, respectively. Capital Lease In July 2016, we entered into a five year capital lease agreement for certain equipment in our Florida manufacturing facility. The current portion of the capital lease obligation is included in Other Current Liabilities and the noncurrent portion is included in Other Liabilities. The following is a schedule of future minimum lease payments due under the capital lease obligation as of December 31, 2018 (in thousands): Year Ended December 31, 2019 $ 33 2020 34 2021 35 2022 9 2023 — Total capital lease obligation 111 Less: amount representing interest (33 ) Present value of net minimum capital lease payments 78 Less: current portion (17 ) Total noncurrent capital lease obligation $ 61 Asset Retirement Obligation We recognized the estimated fair value of our asset retirement obligation related to our office space in London, United Kingdom in long-term liabilities in November 2018. The fair value of the asset retirement obligation is also capitalized as construction in progress. The fair value of the asset retirement obligation was estimated by discounting projected cash flows over the estimated life of the related assets using our credit adjusted risk-free rate. Our asset retirement obligation consists of a contractual requirement to remove the tenant improvements at our office space in London, United Kingdom and restore the lease office space to a condition as specified in the lease agreement. The following is the activity for our asset retirement obligation included in long-term liabilities (in thousands): Balance as of December 31, 2017 $ — Liabilities incurred during the year 88 Balance as of December 31, 2018 $ 88 Purchase Commitments We purchase standard 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 not allowed to sell several peanut flour products to any third party worldwide for use in 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 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 not occur prior to 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 $3.3 million over a term of ten years. In December 2018, we entered into an exclusive supply agreement for egg protein with Michael Foods, Inc. Pursuant to the agreement, we have exclusive access to the clinical and commercial use of Michael Foods’ egg products for any egg allergy treatment, prevention or cure for a period of up to 15 years beyond the potential approval of AR201. 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 our 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 directors’ and officers’ 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.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Convertible Preferred Stock As of December 31, 2018 and 2017, we had authorized 10,000,000 shares of convertible preferred stock, and no shares of convertible preferred stock were issued and outstanding. Common Stock On November 23, 2016, we issued and sold 7,522,084 shares of our common stock, par value $0.0001 per share, for an aggregate cash purchase price of $145.0 million, pursuant to the Purchase Agreement, dated November 2, 2016, by and between us and Nestle Health Science. In connection with the closing of the Equity Investment, we and Nestle Health Science entered into a Registration Rights Agreement and a Standstill Agreement. Under the terms of the Registration Rights Agreement, upon the written request of Nestle Health Science, we shall prepare and file with the Securities and Exchange Commission, or the Commission, a registration statement covering the resale of all the shares sold to Nestle Health Science that are not then registered on an existing and effective registration statement for an offering to be made on a continuous basis pursuant Commission Rule 415. Additionally, we shall use commercially reasonable efforts to cause such registration statement filed under the Registration Rights Agreement to be declared effective under the Securities Act of 1933, as amended, within certain defined time limits and to keep such registration statement continuously effective for a period of potentially three years from the original effect date of such registration statement. Under the terms of the Standstill Agreement, Nestle Health Science is prohibited from entering into transactions with the shares purchased in the Equity Investment, as well as to enter into any transactions with any of our assets, without prior written consent of a majority of the members of our board of directors until the later of the end of the term of the Collaboration Agreement and November 23, 2018. In November 2018, we entered into an extension of the strategic collaboration on similar terms and issued and sold an additional 3,237,529 shares of our common stock in a private placement at a price of $30.27 per share for aggregate proceeds of $98.0 million, increasing Nestlé Health Science’s ownership of Aimmune to approximately 19%. The transaction documents include the extension of the registration rights, standstill rights and market standoff provisions. We are not subject to any partnership, collaboration, or negotiation restrictions under the extension agreements. In addition, we retain all rights to our current and future pipeline assets. 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, including the closing of the full exercise of the underwriters’ option to purchase an additional 825,000 shares of common stock, We issued Archer Daniels Midland Company 300,000

Stock-based Awards

Stock-based Awards	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	7. Stock-Based Awards Equity Plans In July 2015, we adopted the 2015 Stock Plan, or the 2015 Plan. Under the 2015 Plan, 4,681,544 shares of our common stock were initially reserved for the issuance of stock options and restricted stock to employees, directors, and consultants under terms and provisions established by the Board of Directors, or the Board, and approved by our stockholders. As of December 31, 2018 and December 31, 2017 there were 4,364,963 and 4,313,423 and shares available for future grant, respectively. Under the terms of the 2015 Plan, options may be granted at an exercise price not less than fair market value. For employees holding more than 10% of the voting rights of all classes of stock, the exercise prices for incentive stock options may not be less than 110% of fair market value, as determined by the Board. The terms of options granted under the 2015 Plan may not exceed ten years. All options issued to date have had a ten-year life. To date, options granted generally vest in three ways: 1) over four years at a rate of 25% upon the first anniversary of the issuance date and 1/48 th th th In August 2015, we adopted the 2015 ESPP, which commenced on January 1, 2018. Under the 2015 ESPP our employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2015 ESPP generally provides for offering periods of six months in duration with purchase periods ending on either May 15 or November 15. Contributions under the 2015 ESPP are limited to a maximum of 15% of an employee’s eligible compensation. ESPP purchases are settled with common stock from the ESPP’s previously authorized and available pool of shares. We issued 41,030 shares at a weighted average price of $25.28 per share during the year ended December 31, 2018. As of December 31, 2018, 1,784,363 shares under the ESPP remain available for purchase. Our 2013 Stock Plan, or the 2013 Plan, which was originally adopted during January 2013, was terminated upon consummation of our IPO in August 2015. As a terminated plan, no further options can be granted from the 2013 Plan, and no further shares are reserved for issuance under the 2013 Plan. Prior to its termination, the 2013 Plan allowed employees to exercise a stock option in exchange for cash before the requisite service is 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 December 31, 2018 and December 31, 2017, 0 and 46,973 unvested shares were legally issued and outstanding, respectively. In connection with these unvested shares, we have recorded an early exercise liability as of December 31, 2018 and 2017 of $0 and $7,000, respectively, of which $0 and $7,000 is included in other current liabilities as of December 31, 2018 and 2017, respectively, with the remaining was included in other liabilities in the consolidated balance sheets. These shares are excluded from basic and diluted net loss per share until our repurchase right lapses and the shares are no longer subject to the repurchase feature. Activity under the 2015 Plan and 2013 Plan related to options 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 2,417,500 $ 31.21 Options exercised and shares vested (1,190,721 ) $ 8.14 Options cancelled (722,777 ) $ 22.64 Balance, December 31, 2018 7,133,113 $ 20.08 7.6 $ 27,413 Options vested and expected to vest as of December 31, 2018 6,765,252 $ 19.70 7.6 $ 28,518 Options exercisable as of December 31, 2018 3,461,490 $ 13.92 7.8 $ 34,599 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 December 31, 2018. The 2013 Plan provided for early exercise, therefore, all of the outstanding stock options issued under that plan are exercisable. The total intrinsic value of options exercised during the year ended December 31, 2018 was $9.7 million. Restricted stock unit, or RSU, activity under the 2015 Plan is set forth below: Shares Weighted Grant Date Fair Value Unvested Balance, December 31, 2017 16,638 $ 35.41 Awarded 346,962 33.44 Released (4,160 ) 39.55 Forfeited (49,593 ) 35.45 Unvested Balance, December 31, 2018 309,847 $ 33.37 Fair Value of Equity Awards 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 grant date fair value of stock options granted were as follows: Year Ended December 31, 2018 2017 2016 Expected volatility 67.8 % 73.1 % 74.5 % Risk-free interest rate 2.5 % 2.0 % 1.7 % Expected dividend yield — — — Expected term (in years) 6.0 6.0 6.0 Weighted average grant date fair value $ 19.53 $ 13.70 $ 10.53 The weighted-average assumptions used to estimate the fair value of ESPP using the Black-Scholes option valuation model were as follows: Year Ended December 31, 2018 2017 2016 Expected volatility 50.2 % — — Risk-free interest rate 2.0 % — — Expected dividend yield — — — Expected term (in years) 0.8 — — Weighted average fair value $ 11.06 — — Stock-Based Compensation Expense Stock-based compensation expense, net of estimated forfeitures, is reflected in the consolidated statements of operations and comprehensive loss (in thousands) as summarized below: Year Ended December 31, 2018 2017 2016 Research and development $ 9,945 $ 5,077 $ 4,838 General and administrative 22,787 11,642 7,803 Total stock-based compensation expense $ 32,732 $ 16,719 $ 12,641 The fair value of options granted is expensed over the vesting period of the options, which is either four years or two years, on a straight-line basis as the services are being provided. No tax benefits were realized from options during the periods. During the years ended December 31, 2018, 2017 and 2016, we recorded $3.3 million, $0.4 million and $0.9 million of stock compensation expense related to the acceleration of certain former executives’ stock options, respectively. As of December 31, 2018, total unrecognized stock-based compensation expense and expected period over which such compensation will be recognized are as follows ($ in thousands): As of December 31, 2018 Stock-options Unrecognized stock compensation expense $ 52,164 Weighted-average remaining vesting period (years) 2.5 RSU Unrecognized stock compensation expense $ 6,820 Weighted-average remaining vesting period (years) 3.0 ESPP Unrecognized stock compensation expense $ 320 Weighted-average remaining vesting period (years) 0.5

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes The following table presents loss before provision for income taxes (in thousands): Year Ended December 31, 2018 2017 2016 Income/(loss) before income taxes Domestic $ (176,085 ) $ (100,107 ) $ (66,210 ) Foreign (34,606 ) (31,162 ) (14,614 ) Total loss before provision for income taxes $ (210,691 ) $ (131,269 ) $ (80,824 ) The federal, state and foreign income tax provisions for the years ended December 31, 2018, 2017 and 2016 are summarized as follows (in thousands): Year Ended December 31, 2018 2017 2016 Current Federal $ — $ — $ — State — — — Foreign 61 56 — Total Current 61 56 — Deferred Federal — — — State — — — Foreign — — — Total Deferred — — — Total provision for income taxes $ 61 $ 56 $ — Income tax expense for the years ended December 31, 2018, 2017 and 2016 differed from the amount expected by applying the statutory federal tax rate to the loss before taxes as summarized below (in thousands): Year Ended December 31, 2018 2017 2016 Federal tax benefit at statutory rate 21.00 % 34.00 % 34.00 % State tax benefit, net of federal benefit 0.82 % 0.54 % 0.39 % Stock compensation (0.30 )% 1.72 % (1.75 )% Change in valuation allowance (19.29 )% (9.31 )% (27.45 )% Research and development credits 1.65 % 1.70 % 2.53 % Foreign income taxed at different rates (3.48 )% (8.11 )% (6.13 )% Impact related to 2017 Tax Act 0.00 % (20.46 )% 0.00 % Other (0.43 )% (0.12 )% (1.59 )% Income tax expense (0.03 )% (0.04 )% 0.00 % The significant components of our deferred taxes are as follows (in thousands): Year Ended December 31, 2018 2017 Deferred tax assets (liabilities): Net operating loss carryforwards $ 70,831 $ 38,626 Start-up costs 683 740 Stock-based compensation 6,387 3,115 Tax credit carryforwards 11,693 6,353 Accruals 1,765 1,153 Other 482 35 Subtotal deferred tax assets 91,841 50,022 Less: valuation allowance (91,520 ) (49,934 ) Total deferred tax assets 321 88 Basis differences in fixed assets (321 ) (88 ) Net deferred income taxes $ — $ — We recognize deferred income taxes for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. We periodically evaluate the positive and negative evidence bearing upon realizability of our deferred tax assets. Based upon the weight of available evidence, which includes our historical operating performance, reported cumulative net losses since inception and difficulty in accurately forecasting our future results, we maintained a full valuation allowance on the net deferred tax assets as of December 31, 2018 and 2017. As of December 31, 2018, we had net operating loss, or NOL, carryforwards for Federal, California and other state income tax purposes of $326.4 12.0 30.2 As of December 31, 2018, we had Federal and California research credit carryforwards of $12.1 million and $3.1 million, respectively. As of December 31, 2017, we had Federal and California research credit carryforward of approximately $6.7 million and $1.6 million, respectively. The Federal research credits will begin to expire in 2031, while the California research credits have no expiration date. Under Section 382 of the Internal Revenue Code of 1986, as amended, or the Code, if a corporation undergoes an “ownership change,” generally defined as a greater than 50 percentage point change (by value) in its equity ownership over a rolling three-year period, the corporation’s ability to use its pre-change net operating loss, or NOL, carryforwards to offset its post-change taxable income may be limited. Limitations may also apply to the utilization of other pre-change tax attributes as a result of an ownership change. Following the equity investment by Nestle Health Science in November 2016, we performed a Section 382 analysis and determined that we experienced multiple ownership changes under Section 382 of the Code prior to July 31, 2017. Utilization of the NOL and tax credit carryforwards are subject to a substantial annual limitation due to the ownership change limitations set forth in Internal Revenue Code Section 382 and similar state provisions. Such annual limitations could impact the utilization of NOL and tax credit carryforwards in the future. We experienced no significant permanent losses of tax attributes due to these ownership changes. In addition, we may experience more ownership changes under Section 382 of the Code as a result of future changes in our stock ownership, some of which may be outside our control. As a result, our ability to utilize NOL carryforwards or other tax attributes, such as research tax credits, in any taxable year may be further limited if we have experienced an ownership change. Tax benefits from uncertain tax positions are recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on technical merits. The amount recognized is measured as the largest amount of tax benefit that is greater than 50% likely of being realized upon effective settlement. The following table summarizes the activity related to our unrecognized benefits (in thousands): Year Ended December 31, 2018 2017 Beginning balance - unrecognized tax benefit, gross $ 1,657 $ 960 Increases related to tax positions taken during a prior year — — Decreases related to a tax position taken during a prior year — (2 ) Increases related to tax positions taken during the current year 1,398 699 Decreases related to settlements with taxing authorities — — Decreases related to expiration of statute of limitations — — Ending balance - unrecognized tax benefits, gross $ 3,055 $ 1,657 At December 31, 2018, the unrecognized tax benefits for uncertain tax positions were offset against the deferred tax assets and would not affect the income tax rate if recognized due to our being in a valuation allowance position. Our policy is to include interest and penalties related to unrecognized tax benefits, if any, within the provision for taxes in the consolidated statements of operations. We did not accrue any interest or penalties for the years ended December 31, 2018 and 2017. We do not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized tax benefits will significantly change within 12 months of December 31, 2018. We file federal, state and foreign income tax returns in jurisdictions with varying statutes of limitations. Due to our NOL carryforwards, our income tax returns remain subject to examination by federal and most state taxing authorities for all tax years.

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	9. Defined Contribution Plan We sponsor a 401(k) Plan, or the 401(k) Plan, which stipulates that eligible employees may contribute to the 401(k) Plan subject to certain limitations. We may match employee contributions in amounts to be determined at our sole discretion. Commencing in 2018, we began a discretionary matching of employee contributions up to a maximum match of $2,000 in any calendar year. We incurred total expenses of $0.2 million for the year ended December 31, 2018.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions 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 years ended December 31, 2018, 2017 and 2016, there were payments of $0.1 million, $0.2 million and less than $0.2 million, respectively, to MyHealthTeams pursuant to such agreement. At December 31, 2018 and 2017, there was $0.1 million and $0 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

Selected Quarterly Results of O

Selected Quarterly Results of Operations (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Results of Operations (Unaudited)	11. Selected Quarterly Results of Operations (Unaudited) The following table presents our unaudited quarterly financial data. Our quarterly results of operations for these periods are not necessarily indicative of our future results of operations. Quarter Ended March 31 June 30 September 30 December 31 2018 (In thousands, except per share data) Operating expenses: Research and development $ 33,446 $ 35,254 $ 31,691 $ 33,029 General and administrative 16,673 18,559 21,285 25,404 Total operating expenses 50,119 53,813 52,976 58,433 Loss from operations (50,119 ) (53,813 ) (52,976 ) (58,433 ) Interest income, net 636 1,294 1,303 1,417 Loss before provision for income taxes (49,483 ) (52,519 ) (51,673 ) (57,016 ) Provision (benefit) for income taxes 17 33 29 (18 ) Net loss $ (49,500 ) $ (52,552 ) $ (51,702 ) $ (56,998 ) Net loss per common share, basic and diluted $ (0.92 ) $ (0.91 ) $ (0.89 ) $ (0.95 ) Quarter Ended March 31 June 30 September 30 December 31 2017 (In thousands, except per share data) Operating expenses: Research and development $ 17,417 $ 22,191 $ 21,063 $ 28,654 General and administrative 8,924 10,813 11,226 12,986 Total operating expenses 26,341 33,004 32,289 41,640 Loss from operations (26,341 ) (33,004 ) (32,289 ) (41,640 ) Interest income, net 471 507 497 530 Loss before provision for income taxes (25,870 ) (32,497 ) (31,792 ) (41,110 ) Provision for income taxes — — — 56 Net loss $ (25,870 ) $ (32,497 ) $ (31,792 ) $ (41,166 ) Net loss per common share, basic and diluted $ (0.52 ) $ (0.65 ) $ (0.63 ) $ (0.81 )

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Event	12. Subsequent Event In January 2019, we entered into a loan agreement with an affiliate of KKR for up to $170.0 million in three tranches. Of the total loan amount, $40.0 million was funded upon closing of the transaction, with $85.0 million to be funded upon FDA approval of AR101 and satisfaction of other customary borrowing conditions, and $45.0 million to be made available at our option in 2020 upon the satisfaction of certain borrowing conditions. The loan can be prepaid at our discretion, at any time, subject to prepayment fees. The weighted-average interest rate will be calculated based on the daily cost of borrowing, reflecting the relevant adjusted London Interbank Offered Rate or Alternate Base Rate plus the applicable margin. We have the option to elect to make interest payments from available funds or make interest payments in kind by capitalizing such interest amounts on the applicable interest payment date by adding the amounts to the outstanding principal amount of the loan. Any capitalized amounts shall thereafter bear interest. The Company has selected to pay in kind and have the interest capitalized for the fiscal quarter ending March 31, 2019.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Preparation	Basis of Preparation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States, or U.S. GAAP, and include the accounts of our wholly-owned subsidiaries. All significant intercompany transactions have been eliminated. We operate in one reportable segment in the United States.
Foreign Currency Translation	Foreign Currency Translation Our functional currency and the functional currency of all of our subsidiaries is the United States dollar. Accordingly, monetary assets and liabilities in the non-functional currency of these subsidiaries are remeasured using exchange rates in effect at the end of the period. Costs in local currency are remeasured using average exchange rates for the period, except for costs related to those balance sheet items that are remeasured using historical exchange rates. The resulting remeasurement gains and losses are included in the consolidated statements of operations and comprehensive loss as incurred and have not been material for all periods presented.
Use of Estimates	Use of Estimates The preparation of the accompanying 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 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.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents, which are carried at estimated fair value, consist primarily of money market funds and certain available-for-sale investments with maturities of three months or less.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker in making decisions regarding resource allocation and assessing performance. We have one operating segment.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject us to a concentration of credit risk consist of cash and cash equivalents and certain investments in money market funds, agency securities, corporate securities, U.S. government securities and commercial paper. Bank deposits are primarily held by a single financial institution and these deposits may exceed insured limits. We are exposed to credit risk in the event of default by the financial institution holding our cash and cash equivalents and issuers of investments that are recorded on our consolidated balance sheets. We mitigate our risk by investing in high-grade instruments and limiting the concentration in any one issuer, which limits our exposure.
Investments	Investments Our investments consist of available-for-sale securities. Investments with original maturities of greater than 90 days but less than one (1) year are classified as short-term on the consolidated balance sheets. Investments with original maturities greater than one (1) year are classified as long-term on the consolidated balance sheets. Our investments in available-for-sale securities are reported at estimated fair value. Available-for-sale securities consist primarily of agency securities, corporate securities, U.S. government securities and commercial paper. Unrealized gains and losses related to changes in the fair value of securities are recognized in accumulated other comprehensive loss, net of tax, on our consolidated balance sheets. Changes in the fair value of available-for-sale securities impact the consolidated statements of operations and comprehensive loss only when such securities are sold or an other-than-temporary impairment is recognized. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. We regularly review our investment portfolio to determine if any security is other-than-temporarily impaired, which would require us to record an impairment charge in the period any such determination is made. We consider factors such as the duration, severity and the reason for the decline in value, the financial condition of the issuer and any changes thereto, 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. Our assessment on whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the respective assets. Maintenance and repairs are charged to the consolidated statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss, if any, is reflected in the consolidated statements of operations and comprehensive loss. The useful lives of property and equipment are as follows: Computer equipment 3 years Furniture and office equipment 4 years Manufacturing equipment 7 years Buildings 25 years Leasehold improvements Shorter of remaining lease terms or useful life
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows expected to result from the use of the asset and its eventual disposition. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired assets. We have not recorded impairment of any long-lived assets in the periods presented.
Leases	Leases Leases related to our corporate headquarters are classified as operating leases. Rent expense is recognized on a straight-line basis over the terms of the leases and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Incentives granted under facilities leases are deferred and recognized as adjustments to rental expense on a straight-line basis over the term of the lease. In June 2015, we signed a lease for a manufacturing facility in Clearwater, Florida. We were considered the deemed owner for accounting purposes. See Note 5, “Commitments and Contingencies,” for further details.
Research and Development	Research and Development We expense research and development costs as incurred. We record accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of pre-clinical studies and clinical trials and contract manufacturing activities. These costs are a significant component of our research and development expenses. We accrue for these costs based on factors such as estimates of the work completed and in accordance with agreements established with our third-party service providers under the service agreements. We make significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, we adjust our accrued liabilities. We have not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed, number of patients enrolled and the rate of patient enrollments may vary from our estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to our accruals could materially affect our results of operations.
Stock-based Compensation	Stock-Based Compensation We recognize compensation costs related to stock options granted to employees and directors based on the estimated fair value of the awards on the date of grant, net of estimated forfeitures. We estimate the grant date fair value using the Black-Scholes option-pricing model. The grant date fair value of the stock-based awards is generally recognized on a straight-line basis over the requisite service period, which is generally the vesting period of the respective awards. The fair value of the shares granted pursuant to the 2015 Employee Stock Purchase Plan (the “2015 ESPP”) In determining the fair value of the stock-based awards used to calculate stock-based compensation expense, we use the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires judgment to determine. • Expected Term. The expected term of stock-based awards represents the weighted average period the stock-based awards are expected to be outstanding. We have opted to use the simplified method for estimating the expected term as provided by the Securities and Exchange Commission Staff Accounting Bulletin, or SAB, 110 as our stock-based awards are considered “plain vanilla”. The simplified method calculates the expected term as the average time-to-vesting and the contractual life of the options. We plan to continue to use the simplified method under SAB 110 until we have sufficient exercise history as a publicly traded company. • Expected Volatility. As we have limited trading history for our common stock, the expected stock price volatility assumption is determined based on the historical volatilities of a group of industry peers as well as the historical volatility of our own common stock since we began trading subsequent to our IPO in August 2015. Industry peers consist of several public companies in the biopharmaceutical industry with comparable characteristics including enterprise value, risk profiles and position within the industry. We intend to continue to consistently apply this process using the same or similar public companies until a sufficient amount of historical information regarding the volatility of our own common stock share price becomes available, or unless circumstances change such that the identified companies are no longer similar to us, in which case, more suitable companies whose share prices are publicly available would be utilized in the calculation. • Risk-Free Interest Rate. The risk-free interest rate is based on the implied yield available on U.S. Treasury zero-coupon issues in effect at the time of grant for periods corresponding with the expected term of the stock-based award. • Expected Dividend Yield. We have never paid dividends on our common stock and have no plans to pay dividends on our common stock. Therefore, we used an expected dividend yield of zero for all years presented Restricted Stock Units, or “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). In addition to the Black-Scholes assumptions, we estimate our forfeiture rate based on an analysis of our actual forfeitures and will continue to evaluate the adequacy of the forfeiture rate based on actual forfeiture experience, analysis of employee turnover behavior and other factors. The impact from any forfeiture rate adjustment would be recognized in full in the period of adjustment, and if the actual number of future forfeitures differs from our estimates, we might be required to record adjustments to stock-based compensation in future periods. We will continue to use judgment in evaluating the expected volatility, expected terms and forfeiture rates utilized for our stock-based compensation expense calculations on a prospective basis.
Income Taxes	Income Taxes We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of reported assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. Tax benefits from uncertain tax positions are recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on technical merits. The amount recognized is measured as the largest amount of tax benefit that is greater than 50% likely of being realized upon effective settlement.
Comprehensive Income or Loss	Comprehensive Income or Loss Comprehensive income or loss is defined as the change in equity during a period from transactions and other events, excluding changes resulting from investments from owners and distributions to owners. Other comprehensive loss includes net loss and unrealized gains and losses on available-for-sale investments.
Net Loss per Share	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: Year Ended December 31, 2018 2017 2016 Stock options 7,133,113 6,629,111 5,429,267 Restricted stock units 309,847 16,638 17,000
Fair Value Measurements	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 orderly transaction between market participants on the measurement date. 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. Financial instruments include cash equivalents, investments, accounts payable, and accrued liabilities. Our cash equivalents and investments are carried at estimated fair value and remeasured on a recurring basis. The carrying value of accounts payable and accrued liabilities approximate their estimated fair value due to the relatively short-term nature of these instruments. 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. In accordance with fair value accounting requirements, companies may choose to measure eligible financial instruments and certain other items at fair value. We have not elected the fair value option for any eligible financial instruments.
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 impact to our 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 impact to our 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 impact to our 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 impact to our 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 ASU 2018-13, Fair Value Measurement: Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In June 2016, the FASB issued ASU 2016-13, Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments after December 15, 2019 and interim periods within those fiscal years. Early adoption is permitted. We are currently evaluating the impact that the adoption of ASU 2016-13 will have on our consolidated financial statements and related disclosures. In February 2016, the FASB, issued ASU 2016-02, Leases (Topic 842) We will adopt the standard effective in the first quarter of 2019 and will not restate comparative periods upon adoption. We will elect a package of practical expedients for leases that commenced prior to January 1, 2019 and will not reassess: (i) whether any expired or existing contracts are or contain leases; (ii) lease classification for any expired or existing leases; and (iii) initial direct costs capitalization for any existing leases. We will make an accounting policy election to keep leases with an initial term of 12 months or less off of the balance sheet. We will recognize those lease payments in the consolidated statements of operations on a straight-line basis over the lease term. We currently expect that our operating lease commitments will be subject to the new standard and recognized as operating lease liabilities and right-of-use assets upon our adoption of Topic 842, which will increase the total assets and total liabilities that we report relative to such amounts prior to adoption. We estimate adoption of the standard will result in the recognition of right of use assets and additional net lease liabilities of approximately $12.0 million, respectively. Refer to Note 5 for further information on our operating lease commitments. We plan to adopt Topic 842 using the alternative modified retrospective approach with the cumulative effect of adoption recognized to retained earnings on January 1, 2019. We do not believe the new standard will have a material impact on our consolidated statements of operations, nor will it have a notable impact on our liquidity. The standard will also have no impact on our debt-covenant compliance under our current agreements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Useful Lives of Property and Equipment	The useful lives of property and equipment are as follows: Computer equipment 3 years Furniture and office equipment 4 years Manufacturing equipment 7 years Buildings 25 years Leasehold improvements Shorter of remaining lease terms or useful life
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: Year Ended December 31, 2018 2017 2016 Stock options 7,133,113 6,629,111 5,429,267 Restricted stock units 309,847 16,638 17,000

Available-for-Sale Securities_2

Available-for-Sale Securities and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of Financial Instruments Measured at Fair Value on a Recurring Basis	The following table sets forth our financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): December 31, 2018 Level 1 Level 2 Level 3 Total Cash and cash equivalents: Cash and money market funds $ 107,511 $ — $ — $ 107,511 Total cash and cash equivalents $ 107,511 $ — $ — $ 107,511 Investments: Agency securities — 17,352 — 17,352 Corporate securities — 54,474 — 54,474 Commercial paper — 5,965 — 5,965 US government securities — 118,630 — 118,630 Total investments $ — $ 196,421 $ — $ 196,421 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 US 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 December 31, 2018 and December 31, 2017 are as are as follows (in thousands): December 31, 2018 Amortized Cost Gross unrealized gains Gross unrealized losses Total fair value Agency securities $ 17,361 $ - $ (9 ) $ 17,352 Corporate securities 54,536 — (62 ) 54,474 Commercial paper 5,965 — — 5,965 US government securities 118,667 14 (51 ) 118,630 Total available-for-sale investments $ 196,529 $ 14 $ (122 ) $ 196,421 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 US 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)	12 Months Ended
	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Summary of Property And Equipment, Net	Property and equipment, net consists of the following (in thousands): Year Ended December 31, 2018 2017 Furniture and office equipment $ 2,221 $ 1,655 Computer equipment 2,073 1,410 Manufacturing equipment 1,733 830 Leased equipment 100 100 Leasehold improvements 4,469 2,685 Buildings 688 688 Construction in progress 18,295 11,490 Property and equipment, gross 29,579 18,858 Less: accumulated depreciation (3,251 ) (1,653 ) Property and equipment, net $ 26,328 $ 17,205
Summary of Accrued Liabilities	Accrued liabilities consist of the following (in thousands): Year Ended December 31, 2018 2017 Compensation and benefits $ 8,912 $ 6,205 Research and development 15,504 12,716 Professional and consulting 4,691 2,370 Other 37 187 Total accrued liabilities $ 29,144 $ 21,478

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Aggregate Minimum Lease Payments	Total future aggregate minimum lease payments under our operating leases, are as follows (in thousands): Year Ended December 31, 2019 $ 3,133 2020 3,330 2021 3,413 2022 3,497 2023 3,378 and thereafter 1,744 Total $ 18,495
Schedule of Future Minimum Lease Payments Due under Capital Lease Obligation	The following is a schedule of future minimum lease payments due under the capital lease obligation as of December 31, 2018 (in thousands): Year Ended December 31, 2019 $ 33 2020 34 2021 35 2022 9 2023 — Total capital lease obligation 111 Less: amount representing interest (33 ) Present value of net minimum capital lease payments 78 Less: current portion (17 ) Total noncurrent capital lease obligation $ 61
Schedule of Activity for Asset Retirement Obligation Included in Long-Term Liabilities	The following is the activity for our asset retirement obligation included in long-term liabilities (in thousands): Balance as of December 31, 2017 $ — Liabilities incurred during the year 88 Balance as of December 31, 2018 $ 88

Stock-based Awards (Tables)

Stock-based Awards (Tables)	12 Months Ended
	Dec. 31, 2018
Summary of Stock Option Activity	Activity under the 2015 Plan and 2013 Plan related to options 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 2,417,500 $ 31.21 Options exercised and shares vested (1,190,721 ) $ 8.14 Options cancelled (722,777 ) $ 22.64 Balance, December 31, 2018 7,133,113 $ 20.08 7.6 $ 27,413 Options vested and expected to vest as of December 31, 2018 6,765,252 $ 19.70 7.6 $ 28,518 Options exercisable as of December 31, 2018 3,461,490 $ 13.92 7.8 $ 34,599
Restricted Stock Unit Activity	Restricted stock unit, or RSU, activity under the 2015 Plan is set forth below: Shares Weighted Grant Date Fair Value Unvested Balance, December 31, 2017 16,638 $ 35.41 Awarded 346,962 33.44 Released (4,160 ) 39.55 Forfeited (49,593 ) 35.45 Unvested Balance, December 31, 2018 309,847 $ 33.37
Schedule of Weighted Average Assumptions to Fair Value of Stock Options	Fair Value of Equity Awards 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 grant date fair value of stock options granted were as follows: Year Ended December 31, 2018 2017 2016 Expected volatility 67.8 % 73.1 % 74.5 % Risk-free interest rate 2.5 % 2.0 % 1.7 % Expected dividend yield — — — Expected term (in years) 6.0 6.0 6.0 Weighted average grant date fair value $ 19.53 $ 13.70 $ 10.53
Summary of Stock-based Compensation Expense Net of Estimated Forfeitures	Stock-Based Compensation Expense Stock-based compensation expense, net of estimated forfeitures, is reflected in the consolidated statements of operations and comprehensive loss (in thousands) as summarized below: Year Ended December 31, 2018 2017 2016 Research and development $ 9,945 $ 5,077 $ 4,838 General and administrative 22,787 11,642 7,803 Total stock-based compensation expense $ 32,732 $ 16,719 $ 12,641
Summary of Unrecognized Stock-Based Compensation Expense and Expected Period	As of December 31, 2018, total unrecognized stock-based compensation expense and expected period over which such compensation will be recognized are as follows ($ in thousands): As of December 31, 2018 Stock-options Unrecognized stock compensation expense $ 52,164 Weighted-average remaining vesting period (years) 2.5 RSU Unrecognized stock compensation expense $ 6,820 Weighted-average remaining vesting period (years) 3.0 ESPP Unrecognized stock compensation expense $ 320 Weighted-average remaining vesting period (years) 0.5
Employee Stock Purchase Plan
Schedule of Weighted Average Assumptions to Fair Value of Stock Options	The weighted-average assumptions used to estimate the fair value of ESPP using the Black-Scholes option valuation model were as follows: Year Ended December 31, 2018 2017 2016 Expected volatility 50.2 % — — Risk-free interest rate 2.0 % — — Expected dividend yield — — — Expected term (in years) 0.8 — — Weighted average fair value $ 11.06 — —

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Loss Before Provision for Income Taxes	The following table presents loss before provision for income taxes (in thousands): Year Ended December 31, 2018 2017 2016 Income/(loss) before income taxes Domestic $ (176,085 ) $ (100,107 ) $ (66,210 ) Foreign (34,606 ) (31,162 ) (14,614 ) Total loss before provision for income taxes $ (210,691 ) $ (131,269 ) $ (80,824 )
Summary of Federal, State and Foreign Income Tax Provisions	The federal, state and foreign income tax provisions for the years ended December 31, 2018, 2017 and 2016 are summarized as follows (in thousands): Year Ended December 31, 2018 2017 2016 Current Federal $ — $ — $ — State — — — Foreign 61 56 — Total Current 61 56 — Deferred Federal — — — State — — — Foreign — — — Total Deferred — — — Total provision for income taxes $ 61 $ 56 $ —
Summary of Income Tax Expense Differed from Statutory Federal Tax Rate to the Loss Before Taxes	Income tax expense for the years ended December 31, 2018, 2017 and 2016 differed from the amount expected by applying the statutory federal tax rate to the loss before taxes as summarized below (in thousands): Year Ended December 31, 2018 2017 2016 Federal tax benefit at statutory rate 21.00 % 34.00 % 34.00 % State tax benefit, net of federal benefit 0.82 % 0.54 % 0.39 % Stock compensation (0.30 )% 1.72 % (1.75 )% Change in valuation allowance (19.29 )% (9.31 )% (27.45 )% Research and development credits 1.65 % 1.70 % 2.53 % Foreign income taxed at different rates (3.48 )% (8.11 )% (6.13 )% Impact related to 2017 Tax Act 0.00 % (20.46 )% 0.00 % Other (0.43 )% (0.12 )% (1.59 )% Income tax expense (0.03 )% (0.04 )% 0.00 %
Significant Components of the Company's Deferred Taxes	The significant components of our deferred taxes are as follows (in thousands): Year Ended December 31, 2018 2017 Deferred tax assets (liabilities): Net operating loss carryforwards $ 70,831 $ 38,626 Start-up costs 683 740 Stock-based compensation 6,387 3,115 Tax credit carryforwards 11,693 6,353 Accruals 1,765 1,153 Other 482 35 Subtotal deferred tax assets 91,841 50,022 Less: valuation allowance (91,520 ) (49,934 ) Total deferred tax assets 321 88 Basis differences in fixed assets (321 ) (88 ) Net deferred income taxes $ — $ —
Summary of Activity Related to Unrecognized Benefits	The following table summarizes the activity related to our unrecognized benefits (in thousands): Year Ended December 31, 2018 2017 Beginning balance - unrecognized tax benefit, gross $ 1,657 $ 960 Increases related to tax positions taken during a prior year — — Decreases related to a tax position taken during a prior year — (2 ) Increases related to tax positions taken during the current year 1,398 699 Decreases related to settlements with taxing authorities — — Decreases related to expiration of statute of limitations — — Ending balance - unrecognized tax benefits, gross $ 3,055 $ 1,657

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 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

Selected Quarterly Results of_2

Selected Quarterly Results of Operations (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Unaudited Quarterly Financial Data	The following table presents our unaudited quarterly financial data. Our quarterly results of operations for these periods are not necessarily indicative of our future results of operations. Quarter Ended March 31 June 30 September 30 December 31 2018 (In thousands, except per share data) Operating expenses: Research and development $ 33,446 $ 35,254 $ 31,691 $ 33,029 General and administrative 16,673 18,559 21,285 25,404 Total operating expenses 50,119 53,813 52,976 58,433 Loss from operations (50,119 ) (53,813 ) (52,976 ) (58,433 ) Interest income, net 636 1,294 1,303 1,417 Loss before provision for income taxes (49,483 ) (52,519 ) (51,673 ) (57,016 ) Provision (benefit) for income taxes 17 33 29 (18 ) Net loss $ (49,500 ) $ (52,552 ) $ (51,702 ) $ (56,998 ) Net loss per common share, basic and diluted $ (0.92 ) $ (0.91 ) $ (0.89 ) $ (0.95 ) Quarter Ended March 31 June 30 September 30 December 31 2017 (In thousands, except per share data) Operating expenses: Research and development $ 17,417 $ 22,191 $ 21,063 $ 28,654 General and administrative 8,924 10,813 11,226 12,986 Total operating expenses 26,341 33,004 32,289 41,640 Loss from operations (26,341 ) (33,004 ) (32,289 ) (41,640 ) Interest income, net 471 507 497 530 Loss before provision for income taxes (25,870 ) (32,497 ) (31,792 ) (41,110 ) Provision for income taxes — — — 56 Net loss $ (25,870 ) $ (32,497 ) $ (31,792 ) $ (41,166 ) Net loss per common share, basic and diluted $ (0.52 ) $ (0.65 ) $ (0.63 ) $ (0.81 )

Formation and Business of the_2

Formation and Business of the Company - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ 56,998	$ 51,702	$ 52,552	$ 49,500	$ 41,166	$ 31,792	$ 32,497	$ 25,870	$ 210,752	$ 131,325	$ 80,824
Net cash used in operating activities									169,128	99,605	$ 56,614
Accumulated deficit	476,234				$ 265,482				476,234	$ 265,482
Cash, cash equivalents and investments	$ 303,900								$ 303,900

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | Segment	1
Number of operating segments | Segment	1
Impairment of any long lived assets	$ 0	$ 0	$ 0
ASU 2016-02
Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset	12,000,000
Operating lease, liability	$ 12,000,000
Minimum
Significant Accounting Policies [Line Items]
Tax benefit recognized upon effective settlement percentage	50.00%
Dividend Yield
Significant Accounting Policies [Line Items]
Alternative investment, measurement input	0

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Computer Equipment
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	3 years
Furniture And Office Equipment
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	4 years
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	7 years
Buildings
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	25 years
Leasehold improvements
Property Plant And Equipment [Line Items]
Useful lives of property and equipment	Shorter of remaining lease terms or useful life

Summary of Significant Accoun_6

Summary of Significant Accounting Policies -Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share	7,133,113	6,629,111	5,429,267
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	309,847	16,638	17,000

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) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments:
Total investments	$ 196,421	$ 143,358
Agency Securities
Investments:
Total investments	17,352	12,718
Corporate Debt Securities
Investments:
Total investments	54,474	29,344
Commercial Paper
Investments:
Total investments	5,965	54,848
U.S. Government Securities
Investments:
Total investments	118,630	46,448
Fair Value Measurements Recurring
Cash and cash equivalents
Total cash and cash equivalents	107,511	73,487
Investments:
Total investments	196,421	108,943
Fair Value Measurements Recurring | Cash and Money Market Funds
Cash and cash equivalents
Total cash and cash equivalents	107,511	39,072
Fair Value Measurements Recurring | Agency Securities
Investments:
Total investments	17,352	12,718
Fair Value Measurements Recurring | Corporate Debt Securities
Cash and cash equivalents
Total cash and cash equivalents		999
Investments:
Total investments	54,474	28,345
Fair Value Measurements Recurring | Commercial Paper
Cash and cash equivalents
Total cash and cash equivalents		33,416
Investments:
Total investments	5,965	21,432
Fair Value Measurements Recurring | U.S. Government Securities
Investments:
Total investments	118,630	46,448
Level 1 | Fair Value Measurements Recurring
Cash and cash equivalents
Total cash and cash equivalents	107,511	39,072
Level 1 | Fair Value Measurements Recurring | Cash and Money Market Funds
Cash and cash equivalents
Total cash and cash equivalents	107,511	39,072
Level 2 | Fair Value Measurements Recurring
Cash and cash equivalents
Total cash and cash equivalents		34,415
Investments:
Total investments	196,421	108,943
Level 2 | Fair Value Measurements Recurring | Agency Securities
Investments:
Total investments	17,352	12,718
Level 2 | Fair Value Measurements Recurring | Corporate Debt Securities
Cash and cash equivalents
Total cash and cash equivalents		999
Investments:
Total investments	54,474	28,345
Level 2 | Fair Value Measurements Recurring | Commercial Paper
Cash and cash equivalents
Total cash and cash equivalents		33,416
Investments:
Total investments	5,965	21,432
Level 2 | Fair Value Measurements Recurring | U.S. Government Securities
Investments:
Total investments	$ 118,630	$ 46,448

Available-for-Sale Securities_4

Available-for-Sale Securities and Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair value assets transfers between level 1 to level 2	$ 0	$ 0
Fair value assets transfers between level 2 to level 1	$ 0	0
Available-for-sale securities, maturities period	1 year
Other than temporary impairment losses	$ 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	Dec. 31, 2018	Dec. 31, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	$ 196,529	$ 143,466
Gross unrealized gains	14	1
Gross unrealized losses	(122)	(109)
Total investments	196,421	143,358
Agency Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	17,361	12,729
Gross unrealized losses	(9)	(11)
Total investments	17,352	12,718
Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	54,536	29,369
Gross unrealized gains		1
Gross unrealized losses	(62)	(26)
Total investments	54,474	29,344
Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	5,965	54,848
Total investments	5,965	54,848
U.S. Government Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Amortized Cost	118,667	46,520
Gross unrealized gains	14
Gross unrealized losses	(51)	(72)
Total investments	$ 118,630	$ 46,448

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 29,579	$ 18,858
Less: accumulated depreciation	(3,251)	(1,653)
Property and equipment, net	26,328	17,205
Furniture and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,221	1,655
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,073	1,410
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,733	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	4,469	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	$ 18,295	$ 11,490

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Depreciation Expense	$ 1,598	$ 966	$ 534

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Compensation and benefits	$ 8,912	$ 6,205
Research and development	15,504	12,716
Professional and consulting	4,691	2,370
Other	37	187
Total accrued liabilities	$ 29,144	$ 21,478

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) $ in Millions	1 Months Ended				12 Months Ended
	Dec. 31, 2018USD ($)	Nov. 30, 2018	Sep. 30, 2018ft²	Jan. 31, 2018shares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jun. 30, 2017ft²	Dec. 31, 2015ft²	Jun. 30, 2015ft²
Commitments And Contingencies [Line Items]
Expected additional operating lease space possession date					Jan. 1, 2018
Lease, termination date					Jun. 30, 2024
Term of capital lease agreement					5 years
Archer Daniels Midland Company
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					$ 3.3
Long-term purchase commitment period					10 years
Archer Daniels Midland Company | 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
Michael Foods Incorporated | Maximum [Member]
Commitments And Contingencies [Line Items]
Long-term purchase commitment period	15 years
Facility Leases
Commitments And Contingencies [Line Items]
Leased area | ft²			5,099
Lease, termination date		Nov. 30, 2023	Feb. 29, 2024
Rent expense under operating leases					$ 3.8	$ 2.4	$ 1.7
Facility Leases | Corporate Headquarters
Commitments And Contingencies [Line Items]
Leased area | ft²								14,841	38,020
Facility Leases | Manufacturing Facility
Commitments And Contingencies [Line Items]
Leased area | ft²										20,000
Term of lease										120 months
Design costs related to building					24.5
Building shell construction cost capitalized	$ 0.7				0.7
Construction in progress for costs incurred by the lessor	$ 17.8				$ 17.8

Commitments and Contingencies_2

Commitments and Contingencies - Future Aggregate Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 3,133
2,020	3,330
2,021	3,413
2,022	3,497
2,023	3,378
and thereafter	1,744
Total	$ 18,495

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due under Capital Lease Obligation (Details) $ in Thousands	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,019	$ 33
2,020	34
2,021	35
2,022	9
Total capital lease obligation	111
Less: amount representing interest	(33)
Present value of net minimum capital lease payments	78
Less: current portion	(17)
Total noncurrent capital lease obligation	$ 61

Commitments and Contingencies_4

Commitments and Contingencies - Schedule of Activity for Asset Retirement Obligation Included in Long-Term Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
Beginning Balance	$ 0
Liabilities incurred during the year	88
Ending Balance	$ 88

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	Nov. 23, 2016	Nov. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders Equity [Line Items]
Preferred stock, shares authorized				10,000,000	10,000,000
Preferred stock, shares issued				0	0
Preferred stock, shares outstanding				0	0
Issuance of common stock upon securities purchase agreement			6,325,000
Common stock, par value				$ 0.0001	$ 0.0001
Proceeds from issuance of common stock				$ 288,435,000	$ 6,520,000	$ 145,000,000
Underwritten public offering price per share			$ 32
Proceeds from issuance of common stock			$ 190,400,000
Common Stock
Stockholders Equity [Line Items]
Issuance common stock upon initial public offering (in shares)				6,325,000
Underwriter [Member]
Stockholders Equity [Line Items]
Purchase of additional shares of common stock			825,000
NHScUS
Stockholders Equity [Line Items]
Issuance of common stock upon securities purchase agreement				937,500
Ownership percentage		19.00%
Proceeds from issuance of common stock				$ 30,000,000
NHScUS | Purchase Agreement
Stockholders Equity [Line Items]
Issuance of common stock upon securities purchase agreement	7,522,084
Common stock, par value	$ 0.0001
Proceeds from issuance of common stock	$ 145,000,000
NHScUS | Private Placement
Stockholders Equity [Line Items]
Issuance common stock upon initial public offering (in shares)		3,237,529
Share issued price		$ 30.27
Proceeds from issuance of common stock		$ 98,000,000
Archer Daniels Midland Company | Common Stock | Restricted Stock
Stockholders Equity [Line Items]
Shares issued				300,000
Vesting period				3 years 6 months
Convertible Preferred Stock
Stockholders Equity [Line Items]
Preferred stock, shares authorized				10,000,000	10,000,000
Preferred stock, shares issued				0	0
Preferred stock, shares outstanding				0	0

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2015	Jul. 31, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested shares outstanding			7,133,113	6,629,111
Intrinsic Value of option exercised			$ 9,700,000
Expected dividend yield			0.00%
Number of shares available for grant			2,417,500
Former Executive
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Accelerated stock compensation expense			$ 3,300,000	$ 400,000	$ 900,000
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Tax benefit realized from option			$ 0
Maximum | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			4 years
Minimum | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			2 years
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance		4,681,544
Number of shares available for future grant			4,364,963	4,313,423
Minimum percentage of voting rights of all classes of stock		10.00%
Percentage of statutory stock options		110.00%
Options expiration period		10 years
2015 Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options expiration period		10 years
2015 Plan | First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period		4 years
Option vesting rights, percentage		25.00%
2015 Plan | Thereafter
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting rights, percentage		2.08%
2015 Plan | Over Two Years
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period		2 years
Option vesting rights, percentage		4.17%
2015 Plan | Over Four Years
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period		4 years
Option vesting rights, percentage		2.08%
2015 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of statutory stock options	85.00%
Description of ESPP			Under the 2015 ESPP our employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2015 ESPP generally provides for offering periods of six months in duration with purchase periods ending on either May 15 or November 15. Contributions under the 2015 ESPP are limited to a maximum of 15% of an employee’s eligible compensation.
Percentage of maximum contributions employee's eligible compensation	15.00%
ESPP offering period	6 months
Description of offering period closing dates			The 2015 ESPP generally provides for offering periods of six months in duration with purchase periods ending on either May 15 or November 15.
Shares issued under the plan			41,030
Weighted average price per share			$ 25.28
Employee stock purchase plan remain available for purchase			1,784,363
2013 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, shares reserved for issuance			0			0
Number of shares available for future grant			0
Unvested shares issued			0	46,973
Unvested shares outstanding			0	46,973
Exercise liability			$ 0	$ 7,000
Exercise liability current			$ 0	$ 7,000

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 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	2,417,500
Number of Options and Unvested Shares, Options exercised and shares vested	(1,190,721)
Number of Options and Unvested Shares, Options cancelled	(722,777)
Number of Options and Unvested Shares, Ending Balance	7,133,113	6,629,111
Number of Options and Unvested Shares, Options vested and expected to vest as of December 31, 2018	6,765,252
Number of Options and Unvested Shares, Options exercisable as of December 31, 2018	3,461,490
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Beginning Balance	$ 14.15
Weighted-Average Exercise Price, Options granted	31.21
Weighted-Average Exercise Price, Options exercised and shares vested	8.14
Weighted-Average Exercise Price, Options cancelled	22.64
Weighted-Average Exercise Price, Ending Balance	20.08	$ 14.15
Weighted-Average Exercise Price, Options vested and expected to vest as of December 31, 2018	19.70
Weighted-Average Exercise Price, Options exercisable as of December 31, 2018	$ 13.92
Weighted Average Remaining Contractual Life (in years)
Weighted Average Remaining Contractual Life (in years), Balance	7 years 7 months 6 days	8 years 2 months 12 days
Weighted Average Remaining Contractual Life, Options vested and expected to vest as of December 31, 2018	7 years 7 months 6 days
Weighted Average Remaining Contractual Life, Options exercisable as of December 31, 2018	7 years 9 months 18 days
Aggregate Intrinsic Value, Balance	$ 27,413	$ 156,900
Aggregate Intrinsic Value, Options vested and expected to vest as of December 31, 2018	28,518
Aggregate Intrinsic Value, Options exercisable as of December 31, 2018	$ 34,599

Stock-Based Awards - Restricted

Stock-Based Awards - Restricted Stock Unit Activity (Details) - 2015 Plan - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 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	(49,593)
Unvested Shares, Ending Balance | shares	309,847
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	35.45
Weighted Average Grant Date Fair Value, Ending Balance | $ / shares	$ 33.37

Stock-Based Awards - Schedule o

Stock-Based Awards - Schedule of Weighted Average Assumptions to Fair Value of Stock Options (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	67.80%	73.10%	74.50%
Risk-free interest rate	2.50%	2.00%	1.70%
Expected dividend yield	0.00%
Expected term (in years)	6 years	6 years	6 years
Weighted average grant date fair value	$ 19.53	$ 13.70	$ 10.53
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	50.20%
Risk-free interest rate	2.00%
Expected term (in years)	9 months 18 days
Weighted average grant date fair value	$ 11.06

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of Stock-based Compensation Expense Net of Estimated Forfeitures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 32,732	$ 16,719	$ 12,641
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	9,945	5,077	4,838
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 22,787	$ 11,642	$ 7,803

Stock-Based Awards - Summary _3

Stock-Based Awards - Summary of Unrecognized Stock-Based Compensation Expense and Expected Period (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock compensation expense	$ 52,164
Weighted-average remaining vesting period (years)	2 years 6 months
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average remaining vesting period (years)	3 years
Unrecognized stock compensation expense	$ 6,820
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average remaining vesting period (years)	6 months
Unrecognized stock compensation expense	$ 320

Income Taxes - Schedule of Loss

Income Taxes - Schedule of Loss Before Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income/(loss) before income taxes
Domestic									$ (176,085)	$ (100,107)	$ (66,210)
Foreign									(34,606)	(31,162)	(14,614)
Loss before provision for income taxes	$ (57,016)	$ (51,673)	$ (52,519)	$ (49,483)	$ (41,110)	$ (31,792)	$ (32,497)	$ (25,870)	$ (210,691)	$ (131,269)	$ (80,824)

Income Taxes - Summary of Feder

Income Taxes - Summary of Federal, State and Foreign Income Tax Provisions (Details) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Current
Foreign						$ 61	$ 56
Total Current						61	56
Deferred
Total provision for income taxes	$ (18)	$ 29	$ 33	$ 17	$ 56	$ 61	$ 56

Income Taxes - Summary of Incom

Income Taxes - Summary of Income Tax Expense Differed from Statutory Federal Tax Rate to the Loss Before Taxes (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal tax benefit at statutory rate	21.00%	34.00%	34.00%
State tax benefit, net of federal benefit	0.82%	0.54%	0.39%
Stock compensation	(0.30%)	1.72%	(1.75%)
Change in valuation allowance	(19.29%)	(9.31%)	(27.45%)
Research and development credits	1.65%	1.70%	2.53%
Foreign income taxed at different rates	(3.48%)	(8.11%)	(6.13%)
Impact related to 2017 Tax Act	0.00%	(20.46%)	0.00%
Other	(0.43%)	(0.12%)	(1.59%)
Income tax expense	(0.03%)	(0.04%)	0.00%

Income Taxes - Significant Comp

Income Taxes - Significant Components of the Company's Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets (liabilities):
Net operating loss carryforwards	$ 70,831	$ 38,626
Start-up costs	683	740
Stock-based compensation	6,387	3,115
Tax credit carryforwards	11,693	6,353
Accruals	1,765	1,153
Other	482	35
Subtotal deferred tax assets	91,841	50,022
Less: valuation allowance	(91,520)	(49,934)
Total deferred tax assets	321	88
Basis differences in fixed assets	$ (321)	$ (88)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Net change in valuation allowance	$ 41,600,000	$ 13,500,000
Interest and penalties related to unrecognized tax benefits	0	0
Federal
Operating Loss Carryforwards [Line Items]
Operating loss, Net	$ 326,400,000	177,900,000
Net operating loss carryforward expiration year	2,031
Research credit carryforwards	$ 12,100,000	6,700,000
Research credit carryforward expiration year	2,031
Other State
Operating Loss Carryforwards [Line Items]
Operating loss, Net	$ 30,200,000	13,700,000
Net operating loss carryforward expiration year	2,030
California
Operating Loss Carryforwards [Line Items]
Operating loss, Net	$ 12,000,000	12,000,000
Net operating loss carryforward expiration year	2,031
Research credit carryforwards	$ 3,100,000	$ 1,600,000

Income Taxes - Summary of Activ

Income Taxes - Summary of Activity Related to Unrecognized Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Beginning balance - unrecognized tax benefit, gross	$ 1,657	$ 960
Decreases related to a tax position taken during a prior year		(2)
Increases related to tax positions taken during the current year	1,398	699
Ending balance - unrecognized tax benefits, gross	$ 3,055	$ 1,657

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018USD ($)
Defined Contribution Plan Disclosure [Line Items]
Total expenses incurred towards discretionary matching contribution by employer	$ 200,000
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Discretionary matching contribution by employer	$ 2,000

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	2 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 190.4
MyHealth Teams
Related Party Transaction [Line Items]
Payments to service agreement		$ 0.1	$ 0.2
Accrued liabilities due		$ 0.1	$ 0
MyHealth Teams | Minimum
Related Party Transaction [Line Items]
Payments to service agreement				$ 0.2

Related Party Transactions - Sc

Related Party Transactions - Schedule of Aggregate Number of Common Stock Sold to Owners (Details) - USD ($)	2 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 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)	12 Months Ended
	Dec. 31, 2018
Related Party Transactions [Abstract]
Aggregate number of shares of common stock sold, ownership percentage of common stock	Owners of More Than 5%

Selected Quarterly Results of_3

Selected Quarterly Results of Operations (Unaudited) - Schedule of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses
Research and development	$ 33,029	$ 31,691	$ 35,254	$ 33,446	$ 28,654	$ 21,063	$ 22,191	$ 17,417	$ 133,420	$ 89,325	$ 54,642
General and administrative	25,404	21,285	18,559	16,673	12,986	11,226	10,813	8,924	81,921	43,949	26,885
Total operating expenses	58,433	52,976	53,813	50,119	41,640	32,289	33,004	26,341	215,341	133,274	81,527
Loss from operations	(58,433)	(52,976)	(53,813)	(50,119)	(41,640)	(32,289)	(33,004)	(26,341)	(215,341)	(133,274)	(81,527)
Interest income, net	1,417	1,303	1,294	636	530	497	507	471
Loss before provision for income taxes	(57,016)	(51,673)	(52,519)	(49,483)	(41,110)	(31,792)	(32,497)	(25,870)	(210,691)	(131,269)	(80,824)
Provision (benefit) for income taxes	(18)	29	33	17	56				61	56
Net loss	$ (56,998)	$ (51,702)	$ (52,552)	$ (49,500)	$ (41,166)	$ (31,792)	$ (32,497)	$ (25,870)	$ (210,752)	$ (131,325)	$ (80,824)
Net loss per common share, basic and diluted	$ (0.95)	$ (0.89)	$ (0.91)	$ (0.92)	$ (0.81)	$ (0.63)	$ (0.65)	$ (0.52)	$ (3.67)	$ (2.61)	$ (1.89)

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - Subsequent Event	Jan. 31, 2019USD ($)
Subsequent Event [Line Items]
Loan agreement, proceeds receivable upon closing of transaction	$ 40,000,000
Loan agreement, proceeds receivable upon FDA approval	85,000,000
Loan agreement, remaining borrowing capacity upon certain conditions	45,000,000
Maximum [Member]
Subsequent Event [Line Items]
Loan agreement, maximum borrowing capacity	$ 170,000,000