Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (89,380)	$ (91,579)	$ (54,780)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	17,362	16,857	9,694
Depreciation and amortization expense	7,259	4,205	728
Loss from revaluation of preferred stock warrant liability			7
Non-cash interest expense	3		367
Accretion of discount on investments	(216)	(386)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	31	(2,598)	(2,470)
Deferred revenue	(11,888)	108,814
Accounts payable	(856)	3,581	2,682
Accrued expenses and other liabilities	2,162	5,027	2,928
Net cash provided by (used in) operating activities	(75,523)	43,921	(40,844)
Cash flows from investing activities:
Purchases of property and equipment	(4,676)	(21,492)	(4,362)
Purchases of investments	(96,534)	(290,594)	(267,761)
Sales and maturities of investments	158,312	246,494	136,390
Changes in restricted cash	(113)	139	(1,400)
Net cash provided by (used in) investing activities	56,989	(65,453)	(137,133)
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs			(24)
Proceeds from exercise of stock options and common stock warrants	116	2,138	262
Proceeds from issuance of common stock upon completion of initial public offering			143,015
Payments for repurchase of common stock	(33)
Repayment of notes payable			(2,600)
Payments of initial public offering costs			(2,928)
Net cash provided by financing activities	83	2,138	137,725
Net increase (decrease) in cash and cash equivalents	(18,451)	(19,394)	(40,252)
Cash and cash equivalents at beginning of year	54,539	73,933	114,185
Cash and cash equivalents at end of year	36,088	54,539	73,933
Supplemental disclosure of cash flow information:
Cash paid for interest	1	109	163
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 877	$ 1,650	$ 2,953