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, 2017	$ 38,940	$ 115	$ 760,225	$ (97)	$ (721,303)
Balance (in shares) at Dec. 31, 2017		115,555
Net loss	(13,885)				(13,885)
Other comprehensive income (loss)	(329)			(329)
Issuance of common stock from public offering, net of offering costs	57,262	$ 15	57,247
Issuance of common stock from public offering, net of offering costs (in shares)		14,039
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	1,295	$ 1	1,294
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		951
Stock-based compensation	2,315		2,315
Balance at Mar. 31, 2018	85,598	$ 131	821,081	(426)	(735,188)
Balance (in shares) at Mar. 31, 2018		130,545
Balance at Dec. 31, 2018	84,519	$ 136	863,531	(281)	(778,867)
Balance (in shares) at Dec. 31, 2018		136,853
Net loss	(18,792)				(18,792)
Other comprehensive income (loss)	211			211
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
Balance at Mar. 31, 2019	$ 69,500	$ 138	$ 867,091	$ (70)	$ (797,659)
Balance (in shares) at Mar. 31, 2019		137,818