CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income
Beginning balance at Dec. 31, 2019	$ 186,655	$ 9	$ 647,078	$ (460,452)	$ 20
Beginning balance (shares) at Dec. 31, 2019		88,817,741
Issuance of common stock under employee stock purchase plan	375		375
Issuance of common stock under employee stock purchase plan (shares)		75,804
Issuance of common stock for services	310		310
Issuance of common stock for services (shares)		42,403
Issuance of common stock upon exercise of options	420		420
Issuance of common stock upon exercise of options (shares)		204,615
Stock-based compensation	5,622		5,622
Unrealized gains on available-for-sale securities	297				297
Net loss	(47,329)			(47,329)
Ending balance at Jun. 30, 2020	146,350	$ 9	653,805	(507,781)	317
Ending balance (shares) at Jun. 30, 2020		89,140,563
Beginning balance at Mar. 31, 2020	167,757	$ 9	650,617	(482,825)	(44)
Beginning balance (shares) at Mar. 31, 2020		89,035,096
Issuance of common stock for services	310		310
Issuance of common stock for services (shares)		42,403
Issuance of common stock upon exercise of options	204		204
Issuance of common stock upon exercise of options (shares)		63,064
Stock-based compensation	2,674		2,674
Unrealized gains on available-for-sale securities	361				361
Net loss	(24,956)			(24,956)
Ending balance at Jun. 30, 2020	146,350	$ 9	653,805	(507,781)	317
Ending balance (shares) at Jun. 30, 2020		89,140,563
Beginning balance at Dec. 31, 2020	126,112	$ 9	680,872	(554,765)	(4)
Beginning balance (shares) at Dec. 31, 2020		93,599,975
Issuance of common stock under employee stock purchase plan	478		478
Issuance of common stock under employee stock purchase plan (shares)		102,208
Issuance of common stock upon exercise of options	$ 563		563
Issuance of common stock upon exercise of options (shares)	205,000	205,306
Issuance of common stock upon vesting of restricted stock units (shares)		79,784
Issuance of common stock in at-the-market offering	$ 62,446	$ 1	62,445
Issuance of common stock in at-the-market offering (shares)		8,979,744
Stock-based compensation	6,306		6,306
Unrealized gains on available-for-sale securities	8				8
Net loss	(78,344)			(78,344)
Ending balance at Jun. 30, 2021	117,569	$ 10	750,664	(633,109)	4
Ending balance (shares) at Jun. 30, 2021		102,967,017
Beginning balance at Mar. 31, 2021	130,811	$ 10	718,728	(587,920)	(7)
Beginning balance (shares) at Mar. 31, 2021		98,688,577
Issuance of common stock upon exercise of options	543		543
Issuance of common stock upon exercise of options (shares)		194,799
Issuance of common stock upon vesting of restricted stock units (shares)		44,684
Issuance of common stock in at-the-market offering	28,174		28,174
Issuance of common stock in at-the-market offering (shares)		4,038,957
Stock-based compensation	3,219		3,219
Unrealized gains on available-for-sale securities	11				11
Net loss	(45,189)			(45,189)
Ending balance at Jun. 30, 2021	$ 117,569	$ 10	$ 750,664	$ (633,109)	$ 4
Ending balance (shares) at Jun. 30, 2021		102,967,017