CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (shares) at Dec. 31, 2018		57,554,000
Beginning balance at Dec. 31, 2018	$ 131,275	$ 57	$ 731,306	$ (13)	$ (600,075)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		409,000
Exercise of stock options	1,899	$ 1	1,898
Exercise of common stock warrants (shares)		19,000
Exercise of common stock warrants	64		64
Fair value of common stock warrants at time of exercise	910		910
Stock-based compensation	9,752		9,752
Unrealized gain on short-term investments	50			50
Foreign currency translation gain (loss)	4			4
Net loss	(22,992)				(22,992)
Ending balance (shares) at Mar. 31, 2019		57,982,000
Ending balance at Mar. 31, 2019	120,962	$ 58	743,930	41	(623,067)
Beginning balance (shares) at Dec. 31, 2019		59,396,000
Beginning balance at Dec. 31, 2019	194,979	$ 59	819,626	122	(624,828)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		673,000
Exercise of stock options	11,475	$ 1	11,474
Exercise of common stock warrants (shares)		2,000
Exercise of common stock warrants	7		7
Fair value of common stock warrants at time of exercise	141		141
Stock-based compensation	15,808		15,808
Unrealized gain on short-term investments	42			42
Foreign currency translation gain (loss)	(409)			(409)
Net loss	(14,867)				(14,867)
Ending balance (shares) at Mar. 31, 2020		60,071,000
Ending balance at Mar. 31, 2020	$ 207,176	$ 60	$ 847,056	$ (245)	$ (639,695)