CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2018	$ 84,519	$ 136	$ 863,531	$ (281)	$ (778,867)
Balance (in shares) at Dec. 31, 2018		136,853
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	684	$ 2	682
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		965
Stock-based compensation	2,878		2,878
Other comprehensive income (loss)	211			211
Net loss	(18,792)				(18,792)
Balance at Mar. 31, 2019	69,500	$ 138	867,091	(70)	(797,659)
Balance (in shares) at Mar. 31, 2019		137,818
Balance at Dec. 31, 2019	57,052	$ 144	906,905	114	(850,111)
Balance (in shares) at Dec. 31, 2019		144,291
Issuance of common stock from public offerings, net of offering costs	67,112	$ 18	67,094
Issuance of common stock from public offering, net of offering costs (in shares)		17,867
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	1,193	$ 2	1,191
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		1,803
Stock-based compensation	3,754		3,754
Other comprehensive income (loss)	(237)			(237)
Net loss	(16,466)				(16,466)
Balance at Mar. 31, 2020	$ 112,408	$ 164	$ 978,944	$ (123)	$ (866,577)
Balance (in shares) at Mar. 31, 2020		163,961