CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Accumulated other comprehensive income	Accumulated deficit
Beginning balance at Dec. 31, 2014	$ 28,062	$ 14	$ 260,616	$ 9,521	$ (242,089)
Beginning balance (in shares) at Dec. 31, 2014		13,566,726
Increase (Decrease) in Stockholders' Equity
Net loss	(64,544)				(64,544)
Stock-based compensation expense	10,254		10,254
Exercise of options for cash	552		552
Exercise of options for cash (in shares)		36,566
Issuance of common stock, net of costs	143,293	$ 4	143,289
Issuance of common stock, net of costs (in shares)		4,837,500
Issuance of common stock from ESPP	522		522
Issuance of common stock from ESPP purchase (in shares)		32,645
Net exercise of warrants	$ 0	$ 1	(1)
Net exercise of warrants (in shares)	809,498	809,498
Unrealized gain on investments	$ 37			37
Ending balance at Dec. 31, 2015	118,176	$ 19	415,232	9,558	(306,633)
Ending balance (in shares) at Dec. 31, 2015		19,282,935
Increase (Decrease) in Stockholders' Equity
Net loss	(83,118)				(83,118)
Stock-based compensation expense	10,624		10,624
Exercise of options for cash	240		240
Exercise of options for cash (in shares)		22,132
Issuance of common stock from ESPP	297		297
Issuance of common stock from ESPP purchase (in shares)		28,483
Net exercise of warrants	$ 0
Net exercise of warrants (in shares)	603,545	289,789
Exercise of warrants for cash	$ 2,115	$ 1	2,114
Exercise of warrants for cash (in shares)	313,756	313,756
Unrealized gain on investments	$ (25)			(25)
Ending balance at Dec. 31, 2016	$ 48,309	$ 20	428,507	9,533	(389,751)
Ending balance (in shares) at Dec. 31, 2016	19,937,095	19,937,095
Increase (Decrease) in Stockholders' Equity
Net loss	$ (70,430)				(70,430)
Stock-based compensation expense	6,786		6,786
Exercise of options for cash	$ 399		399
Exercise of options for cash (in shares)	45,573	45,573
Issuance of common stock, net of costs	$ 153,530	$ 8	153,522
Issuance of common stock, net of costs (in shares)		7,941,688
Issuance of common stock from ESPP	144		144
Issuance of common stock from ESPP purchase (in shares)		59,976
Net exercise of warrants	$ 0
Net exercise of warrants (in shares)	638,554	52,825
Exercise of warrants for cash	$ 4,604	$ 1	4,603
Exercise of warrants for cash (in shares)	585,729	585,729
Unrealized gain on investments	$ (54)			(54)
Ending balance at Dec. 31, 2017	$ 143,288	$ 29	594,407	$ 9,479	$ (460,627)
Ending balance (in shares) at Dec. 31, 2017	28,622,886	28,622,886
Increase (Decrease) in Stockholders' Equity
Cumulative effect of accounting change for the adoption of ASU 2016.09			$ 446