Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (38,557)	$ (27,411)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,347	1,781
Depreciation	977	815
Accretion of discounts and amortization of premiums on investments	62
Change in operating assets and liabilities:
Receivable from collaboration partner	45,000
Prepaid expenses and other current assets	(151)	520
Other long-term assets	(149)
Accounts payable	228	(372)
Accrued liabilities	2,029	1,027
Other long-term liabilities	(118)	(75)
Deferred revenue	(16,875)	(10,649)
Net cash used in operating activities	(3,207)	(34,364)
Cash flows from investing activities:
Purchases of investments	(44,044)
Sale of investments	4,000
Purchases of property and equipment	(917)	(854)
Net cash used in investing activities	(40,961)	(854)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of offering costs	133,925
Proceeds from exercise of stock options and employee stock purchase plan	1,668	226
Payments of prior period offering costs	(38)	(153)
Net cash provided by financing activities	135,555	73
Net increase (decrease) in cash and cash equivalents	91,387	(35,145)
Cash and cash equivalents, beginning of period	135,797	112,265
Cash and cash equivalents, end of period	227,184	77,120
Supplemental disclosures of noncash investing and financing activities
Unpaid offering costs included in period-end accounts payable and accrued liabilities	63	461
Vesting of early exercised options and restricted stock	156	226
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 38	$ 7