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	(27,167)				(27,167)
Other comprehensive income (loss)	(320)			(320)
Issuance of common stock from public offering, net of offering costs	69,172	$ 17	69,155
Issuance of common stock from public offering, net of offering costs (in shares)		15,819
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	4,911	$ 1	4,910
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		2,088
Stock-based compensation	4,908		4,908
Balance at Jun. 30, 2018	90,444	$ 133	839,198	(417)	(748,470)
Balance (in shares) at Jun. 30, 2018		133,462
Balance at Mar. 31, 2018	85,598	$ 131	821,081	(426)	(735,188)
Balance (in shares) at Mar. 31, 2018		130,545
Net loss	(13,282)				(13,282)
Other comprehensive income (loss)	9			9
Issuance of common stock from public offering, net of offering costs	11,910	$ 2	11,908
Issuance of common stock from public offering, net of offering costs (in shares)		1,780
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	3,616		3,616
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		1,137
Stock-based compensation	2,593		2,593
Balance at Jun. 30, 2018	90,444	$ 133	839,198	(417)	(748,470)
Balance (in shares) at Jun. 30, 2018		133,462
Balance at Dec. 31, 2018	84,519	$ 136	863,531	(281)	(778,867)
Balance (in shares) at Dec. 31, 2018		136,853
Net loss	(36,354)				(36,354)
Other comprehensive income (loss)	372			372
Issuance of common stock from public offering, net of offering costs	9,836	$ 2	9,834
Issuance of common stock from public offering, net of offering costs (in shares)		1,978
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	1,242	$ 2	1,240
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		1,230
Stock-based compensation	6,017		6,017
Balance at Jun. 30, 2019	65,632	$ 140	880,622	91	(815,221)
Balance (in shares) at Jun. 30, 2019		140,061
Balance at Mar. 31, 2019	69,500	$ 138	867,091	(70)	(797,659)
Balance (in shares) at Mar. 31, 2019		137,818
Net loss	(17,562)				(17,562)
Other comprehensive income (loss)	161			161
Issuance of common stock from public offering, net of offering costs	9,836	$ 2	9,834
Issuance of common stock from public offering, net of offering costs (in shares)		1,978
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	558		558
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		265
Stock-based compensation	3,139		3,139
Balance at Jun. 30, 2019	$ 65,632	$ 140	$ 880,622	$ 91	$ (815,221)
Balance (in shares) at Jun. 30, 2019		140,061