Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flow from operating activities:
Net loss	$ (17,820)	$ (14,762)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Stock-based compensation expense	3,631	1,405
Depreciation	300	316
Amortization of premiums on investments	2	22
Change in operating assets and liabilities:
Receivable from collaboration partner	1,013	45,000
Prepaid expenses and other current assets	(444)	(419)
Other long-term assets	(146)	(105)
Accounts payable	212	629
Accrued liabilities	(1,257)	(1,613)
Prepayment from collaboration partner	3,269
Other long-term liabilities	(43)	(44)
Deferred revenue	(5,331)	(2,410)
Net cash (used in) provided by operating activities	(16,614)	28,019
Cash flow from investing activities:
Purchases of investments	(31,749)	(32,020)
Sales of investments		4,000
Maturities of investments	8,000
Purchases of property and equipment	(1,188)	(185)
Proceeds from sale of equipment	39
Net cash used in investing activities	(24,898)	(28,205)
Cash flow from financing activities:
Proceeds from exercise of stock options and employee stock purchase plan	806	21
Payments of financing-related costs		(38)
Net cash provided by (used in) financing activities	806	(17)
Net decrease in cash, cash equivalents and restricted cash	(40,706)	(203)
Cash, cash equivalents and restricted cash, beginning of period	224,857	136,056
Cash, cash equivalents and restricted cash, end of period	184,151	135,853
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	18	54
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 317	$ 79