CONDENSED 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	$ (65,984)	$ (38,389)	$ (48,555)
Adjustments to reconcile net loss to net cash used in operating activities:
Loss from disposition of assets		4
Non-employee stock option expense			143
Stock-based compensation	10,720	6,847	5,442
Issuance of common stock for services rendered	150	172	122
Accretion of discounts and premiums on investments	94	566	599
Depreciation and amortization expense	746	656	488
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,962)	1,064	(1,889)
Accounts payable, accrued expenses, and other liabilities	1,674	1,988	(1,709)
Deferred revenue	(697)	(1,111)	2,373
Net cash used in operating activities	(55,259)	(28,203)	(42,986)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities		(2,946)	(63,106)
Proceeds from maturity of available-for-sale securities	28,270	32,746	29,420
Proceeds from sale of available-for-sale securities		1,974	999
Purchases of property and equipment	(206)	(408)	(727)
Net cash provided by (used in) investing activities	28,064	31,366	(33,414)
Cash Flows from Financing Activities:
Proceeds from equity financings, net of issuance costs	53,763	49,014	80,599
Proceeds from exercise of common stock warrants and options and employee stock purchases	5,697	2,172	2,546
Payments on note payable	(292)	(261)	(120)
Payments on capital lease	(11)	(7)	(10)
Net cash provided by financing activities	59,157	50,918	83,015
Net increase in cash and cash equivalents	31,962	54,081	6,615
Cash and cash equivalents, beginning of period	80,667	26,586	19,971
Cash and cash equivalents, end of period	$ 112,629	$ 80,667	$ 26,586