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	$ (38,389)	$ (48,555)	$ (38,642)
Adjustments to reconcile net loss to net cash used in operating activities:
Loss from disposition of assets	4		1
Non-employee stock option expense		143	24
Stock-based compensation	6,847	5,442	4,322
Issuance of common stock for services rendered	172	122	82
Accretion of premiums and discounts on investments	566	599	213
Depreciation and amortization expense	656	488	206
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,064	(1,889)	(329)
Accounts payable, accrued expenses, and other liabilities	1,988	(1,709)	2,802
Deferred revenue	(1,111)	2,373
Net cash used in operating activities	(28,203)	(42,986)	(31,321)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(2,946)	(63,106)	(23,623)
Proceeds from maturity of available-for-sale securities	32,746	29,420	4,115
Proceeds from sale of available-for-sale securities	1,974	999
Purchases of property and equipment	(408)	(727)	(1,093)
Net cash provided by (used in) investing activities	31,366	(33,414)	(20,601)
Cash Flows from Financing Activities:
Sale of common stock and warrants, net of issuance costs	49,014	80,599	37,137
Proceeds from issuance of note payable			825
Proceeds from exercise of common stock warrants and options and employee stock purchases	2,172	2,546	8,456
Dividends paid			(798)
Payments on note payable	(261)	(120)
Payments on capital lease	(7)	(10)	(5)
Net cash provided by financing activities	50,918	83,015	45,615
Net increase (decrease) in cash and cash equivalents	54,081	6,615	(6,307)
Cash and cash equivalents, beginning of period	26,586	19,971	26,278
Cash and cash equivalents, end of period	$ 80,667	$ 26,586	$ 19,971