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, 2018	84,519	$ 136	863,531	(281)	(778,867)
Balance (in shares) at Dec. 31, 2018		136,853
Net loss	(36,354)
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
Issuance of common stock from public offerings, 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 and vesting of restricted stock units	558		558
Issuance of common stock from exercise of stock options and vesting of restricted stock units (in shares)		265
Stock-based compensation	3,139		3,139
Other comprehensive income (loss)	161			161
Net loss	(17,562)				(17,562)
Balance at Jun. 30, 2019	65,632	$ 140	880,622	91	(815,221)
Balance (in shares) at Jun. 30, 2019		140,061
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
Balance at Dec. 31, 2019	57,052	$ 144	906,905	114	(850,111)
Balance (in shares) at Dec. 31, 2019		144,291
Net loss	(31,337)
Balance at Jun. 30, 2020	114,525	$ 166	994,682	1,125	(881,448)
Balance (in shares) at Jun. 30, 2020		166,012
Balance at Mar. 31, 2020	112,408	$ 164	978,944	(123)	(866,577)
Balance (in shares) at Mar. 31, 2020		163,961
Issuance of common stock from public offerings, net of offering costs	9,429	$ 1	9,428
Issuance of common stock from public offering, net of offering costs (in shares)		1,471
Issuance of common stock from exercise of stock options and vesting of restricted stock units	1,605	$ 1	1,604
Issuance of common stock from exercise of stock options and vesting of restricted stock units (in shares)		580
Stock-based compensation	4,706		4,706
Other comprehensive income (loss)	1,248			1,248
Net loss	(14,871)				(14,871)
Balance at Jun. 30, 2020	$ 114,525	$ 166	$ 994,682	$ 1,125	$ (881,448)
Balance (in shares) at Jun. 30, 2020		166,012