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 (shares) at Dec. 31, 2016		19,937,095
Beginning balance at Dec. 31, 2016	$ 48,309	$ 20	$ 428,507	$ 9,533	$ (389,751)
Increase (Decrease) in Stockholders' Equity
Net loss	(70,430)				(70,430)
Issuance of common stock and warrants, net of issuance costs (shares)		7,941,688
Issuance of common stock and warrants, net of issuance costs	153,530	$ 8	153,522
Share-based compensation expense	6,786		6,786
Issuance of common stock from ESPP (shares)		59,976
Issuance of common stock from ESPP	144		144
Exercise of options for cash (shares)		45,573
Exercise of options for cash	$ 399		399
Exercise of warrants for cash (shares)	585,729	585,729
Exercise of warrants for cash	$ 4,604	$ 1	4,603
Net exercise of warrants (shares)	638,554	52,825
Unrealized gain (loss) on investments	$ (54)			(54)
Ending balance (shares) at Dec. 31, 2017		28,622,886
Ending balance at Dec. 31, 2017	143,288	$ 29	594,407	9,479	(460,627)
Increase (Decrease) in Stockholders' Equity
Net loss	(98,418)				(98,418)
Issuance of common stock and warrants, net of issuance costs (shares)		3,162,500
Issuance of common stock and warrants, net of issuance costs	130,663	$ 3	130,660
Share-based compensation expense	15,854		15,854
Issuance of common stock from ESPP (shares)		21,536
Issuance of common stock from ESPP	442		442
Exercise of options for cash (shares)		731,935
Exercise of options for cash	9,747	$ 1	9,746
Unrealized gain (loss) on investments	$ 0
Ending balance (shares) at Dec. 31, 2018	32,538,857	32,538,857
Ending balance at Dec. 31, 2018	$ 201,576	$ 33	751,109	9,479	(559,045)
Increase (Decrease) in Stockholders' Equity
Net loss	(213,256)				(213,256)
Issuance of common stock and warrants, net of issuance costs (shares)		4,269,838
Issuance of common stock and warrants, net of issuance costs	327,830	$ 4	327,826
Share-based compensation expense	55,537		55,537
Issuance of common stock from ESPP (shares)		14,488
Issuance of common stock from ESPP	$ 675		675
Exercise of options for cash (shares)	569,146	569,146
Exercise of options for cash	$ 8,473	$ 1	8,472
Adjustments to Additional Paid-in-Capital, Disgorgement of Stockholders' Short-Swing Profits	$ 1,050		1,050
Net exercise of warrants (shares)	2,125,000	2,125,000
Net exercise of warrants		$ 2	(2)
Unrealized gain (loss) on investments	$ 410			410
Ending balance (shares) at Dec. 31, 2019	39,517,329	39,517,329
Ending balance at Dec. 31, 2019	$ 382,295	$ 40	$ 1,144,667	$ 9,889	$ (772,301)