Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flow from operating activities
Net loss	$ (120,324)	$ (97,183)	$ (72,900)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	23,364	16,881	3,513
Depreciation	2,683	1,202	471
Non-cash research and development expense	14,500	10,000
Re-measurement of warrant to purchase redeemable securities		87	241
Change in fair value of preferred stock tranche asset or liability			35,551
Changes in fair value of anti-dilutive protection liability			1,609
Other non-cash items, net	(300)	869	53
Changes in operating assets and liabilities:
Accounts receivable	(591)	931	(1,019)
Prepaid expenses and other current assets	(596)	(1,306)	(373)
Other non-current assets	2	2,246
Accounts payable	(1,515)	3,251	(1,436)
Accrued expenses	(8,334)	11,841	3,526
Deferred revenue	81,707	935	25,321
Other current and non-current liabilities	(13)
Net cash used in operating activities	(9,417)	(50,246)	(5,443)
Cash flow from investing activities
Purchases of property and equipment	(2,059)	(3,493)	(1,431)
Proceeds from the sale of equipment	15	20
Purchases of marketable securities	(375,266)
Proceeds from maturities of marketable securities	193,500
Net cash used in investing activities	(183,810)	(3,473)	(1,431)
Cash flow from financing activities
Proceeds from equipment loan, net of issuance costs			1,500
Proceeds from public offering of common stock, net of issuance costs	154,143	97,488
Proceeds from exercise of stock options	1,755	233
Payments on construction financing lease obligation	(764)	(560)
Proceeds from the issuance of redeemable convertible preferred stock and tranche rights, net of issuance costs			141,711
Payments of notes payable	(600)
Payments of equipment loan principal			(2,000)
Payments of final fee for loan payoff			(80)
Proceeds from the issuance of common stock and restricted stock			6
Payments of initial public offering costs			(1,746)
Net cash provided by financing activities	154,534	97,161	139,391
Net increase (decrease) in cash, cash equivalents, and restricted cash	(38,693)	43,442	132,517
Cash, cash equivalents, and restricted cash, beginning of period	186,942	143,500	10,983
Cash, cash equivalents, and restricted cash, end of period	148,249	186,942	143,500
Supplemental disclosure of cash and non-cash activities:
Accretion of redeemable convertible preferred stock to redemption value		47	394
Fixed asset additions included in accounts payable and accrued expenses	623	130	58
Construction financing lease obligation		35,941
Conversion of anti-dilutive protection liability to common stock			1,936
Reclassification of warrants to additional paid in capital		376
Conversion of preferred stock to common stock upon closing of the initial public offering		199,962
Reclassification of liability for common stock subject to repurchase	11	11	17
Interest paid	13	$ 465	91
Offering expenses included in accounts payable and accrued expenses	235		502
Reclassification of tranche rights upon issuance of redeemable convertible preferred stock			$ 37,038
Issuance of common stock for settlement of notes payable	$ 14,823