Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities:
Net loss	$ (24,384)	$ (18,180)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	2,746	1,018
Depreciation	634	531
Accretion of discounts and amortization of premiums on investments	41
Change in operating assets and liabilities:
Receivable from collaboration partner	45,000
Prepaid expenses and other current assets	152	216
Other long-term assets	(80)
Accounts payable	(488)	(436)
Accrued liabilities	1,470	50
Other long-term liabilities	(86)	(43)
Deferred revenue	(11,250)	(7,100)
Net cash provided by (used in) operating activities	13,755	(23,944)
Cash flows from investing activities:
Purchases of investments	(36,022)
Sale of investments	4,000
Purchases of property and equipment	(617)	(766)
Net cash used in investing activities	(32,639)	(766)
Cash flows from financing activities:
Proceeds from exercise of stock options and employee stock purchase plan	430	213
Payments of financing-related costs	(38)	(153)
Net cash provided by financing activities	392	60
Net decrease in cash and cash equivalents	(18,492)	(24,650)
Cash and cash equivalents, beginning of period	135,797	112,265
Cash and cash equivalents, end of period	117,305	87,615
Supplemental disclosures of noncash investing and financing information
Vesting of early exercised options and restricted stock	99	180
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 218	$ 12