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	$ (84,979)	$ (95,817)	$ (82,177)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	311	215	194
Stock-based compensation expense	13,349	10,923	10,699
Changes in assets and liabilities
Prepaid expenses and other assets	3,940	(1,626)	317
Accounts payable	142	(315)	413
Accrued and other liabilities	(8,805)	5,742	1,490
Income taxes payable	(247)	247
Upfront payment from collaboration partner	(40,000)	40,000
Net cash used in operating activities	(116,289)	(40,631)	(69,064)
Cash flows from investing activities
Purchase of property and equipment	(745)	(90)
Leasehold improvements for build-to-suit asset	(3,558)
Security deposit for facility lease			(42)
Change in restricted cash	(2,383)
Net cash used in investing activities	(6,686)	(90)	(42)
Cash flows from financing activities
Proceeds from issuance of common stock in follow-on offering, net of issuance costs		59,136	80,209
Proceeds from issuance of common stock in connection with employee benefit plans	2,968	669	400
Net cash provided by financing activities	2,968	59,805	80,609
Net (decrease) increase in cash and cash equivalents	(120,007)	19,084	11,503
Cash and cash equivalents at beginning of period	201,153	182,069	170,566
Cash and cash equivalents at end of period	81,146	$ 201,153	$ 182,069
Supplemental disclosure of noncash items
Build-to-suit lease transaction	$ 5,428