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	$ (95,817)	$ (82,177)	$ (57,513)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	215	194	107
Loss on disposition of assets			26
Stock-based compensation expense	10,923	10,699	4,641
Remeasurement of convertible preferred stock call option liability			9,560
Remeasurement of convertible preferred stock warrant liability			2,279
Changes in assets and liabilities
Prepaid expenses and other assets	(1,626)	317	(1,695)
Accounts payable	(315)	413	944
Accrued and other liabilities	5,742	1,490	1,998
Income taxes payable	247
Upfront payment from collaboration partner	40,000
Net cash used in operating activities	(40,631)	(69,064)	(39,653)
Cash flows from investing activities
Purchase of property and equipment	(90)		(827)
Security deposit for facility lease		(42)	55
Net cash used in investing activities	(90)	(42)	(772)
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs			132,137
Proceeds from issuance of common stock in follow-on offering, net of issuance costs	59,136	80,209
Proceeds from issuance of convertible preferred stock, net of issuance costs			64,793
Proceeds from exercise of convertible preferred stock warrants			572
Proceeds from issuance of common stock in connection with employee benefit plans	669	400	201
Net cash provided by financing activities	59,805	80,609	197,703
Net increase in cash and cash equivalents	19,084	11,503	157,278
Cash and cash equivalents at beginning of period	182,069	170,566	13,288
Cash and cash equivalents at end of period	$ 201,153	$ 182,069	170,566
Conversion of preferred stock call option liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital			9,581
Conversion of preferred stock warrant liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital			2,752
Conversion of preferred stock to common stock [Member]
Supplemental disclosure of noncash items
Conversion to common stock and additional paid in capital			$ 122,290