Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (91,579)	$ (54,780)	$ (16,709)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	16,857	9,694	2,068
Depreciation and amortization expense	4,205	728	190
Loss from revaluation of preferred stock warrant liability		7	1,418
Licensing fees paid in common stock warrant			317
Non-cash interest expense		367	81
Accretion of discount on investments	(386)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,598)	(2,470)	(7)
Deferred revenue	108,814
Accounts payable	3,581	2,682	810
Accrued expenses and other liabilities	5,027	2,928	1,474
Net cash provided by (used in) operating activities	43,921	(40,844)	(10,358)
Cash flows from investing activities:
Purchases of property and equipment	(21,492)	(4,362)	(1,001)
Purchases of investments	(290,594)	(267,761)
Sales and maturities of investments	246,494	136,390
Changes in restricted cash	139	(1,400)	(102)
Net cash used in investing activities	(65,453)	(137,133)	(1,103)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		(24)	123,203
Proceeds from issuance of notes payable and preferred stock warrant, net of issuance costs			2,000
Proceeds from exercise of stock options and common stock warrants	2,138	262	5
Proceeds from issuance of common stock upon completion of initial public offering		143,015
Proceeds from issuance of common stock and restricted common stock			5
Repayment of notes payable		(2,600)	(400)
Payments of initial public offering costs		(2,928)	(821)
Net cash provided by financing activities	2,138	137,725	123,992
Net increase (decrease) in cash and cash equivalents	(19,394)	(40,252)	112,531
Cash and cash equivalents at beginning of year	73,933	114,185	1,654
Cash and cash equivalents at end of year	54,539	73,933	114,185
Supplemental disclosure of cash flow information:
Cash paid for interest	109	163	122
Supplemental disclosure of non-cash investing and financing activities:
Accretion of convertible preferred stock to redemption value			1,291
Deferred offering costs included in accounts payable			863
Property and equipment purchases included in accounts payable and accrued expenses	$ 1,650	$ 2,953	$ 101