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, 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 offerings, 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 offerings, 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
Balance at Dec. 31, 2020	103,806	$ 168	1,012,932	674	(909,968)
Balance (in shares) at Dec. 31, 2020		168,170
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases	1,171		1,171
Issuance of common stock from exercise of stock options, vesting of restricted stock units, and ESPP purchases (in shares)		2,621
Stock-based compensation	5,333		5,333
Other comprehensive income (loss)	(218)			(218)
Net loss	(17,457)				(17,457)
Balance at Mar. 31, 2021	92,635	$ 168	1,019,436	456	(927,425)
Balance (in shares) at Mar. 31, 2021		170,791
Balance at Dec. 31, 2020	103,806	$ 168	1,012,932	674	(909,968)
Balance (in shares) at Dec. 31, 2020		168,170
Net loss	(32,818)
Balance at Jun. 30, 2021	84,237	$ 172	1,026,534	317	(942,786)
Balance (in shares) at Jun. 30, 2021		171,572
Balance at Mar. 31, 2021	92,635	$ 168	1,019,436	456	(927,425)
Balance (in shares) at Mar. 31, 2021		170,791
Issuance of common stock from exercise of stock options and vesting of restricted stock units	1,294	$ 4	1,290
Issuance of common stock from exercise of stock options and vesting of restricted stock units (in shares)		781
Stock-based compensation	5,808		5,808
Other comprehensive income (loss)	(139)			(139)
Net loss	(15,361)				(15,361)
Balance at Jun. 30, 2021	$ 84,237	$ 172	$ 1,026,534	$ 317	$ (942,786)
Balance (in shares) at Jun. 30, 2021		171,572