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	834		834
Issuance of common stock under employee stock purchase plan (shares)		169,931
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	759		759
Issuance of common stock upon exercise of options (shares)		346,806
Issuance of common stock upon vesting of restricted stock units (shares)		866,528
Stock-based compensation	8,149		8,149
Unrealized (losses) gains on available-for-sale securities	70				70
Net loss	(65,437)			(65,437)
Ending balance at Sep. 30, 2020	131,340	$ 9	657,130	(525,889)	90
Ending balance (shares) at Sep. 30, 2020		90,243,409
Beginning balance at Jun. 30, 2020	146,350	$ 9	653,805	(507,781)	317
Beginning balance (shares) at Jun. 30, 2020		89,140,563
Issuance of common stock under employee stock purchase plan	459		459
Issuance of common stock under employee stock purchase plan (shares)		94,127
Issuance of common stock upon exercise of options	339		339
Issuance of common stock upon exercise of options (shares)		142,191
Issuance of common stock upon vesting of restricted stock units (shares)		866,528
Stock-based compensation	2,527		2,527
Unrealized (losses) gains on available-for-sale securities	(227)				(227)
Net loss	(18,108)			(18,108)
Ending balance at Sep. 30, 2020	131,340	$ 9	657,130	(525,889)	90
Ending balance (shares) at Sep. 30, 2020		90,243,409
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	819		819
Issuance of common stock under employee stock purchase plan (shares)		386,664
Issuance of common stock for services	190		190
Issuance of common stock for services (shares)		25,989
Issuance of common stock upon exercise of options	$ 584		584
Issuance of common stock upon exercise of options (shares)	331,000	331,310
Issuance of common stock upon vesting of restricted stock units (shares)		114,194
Taxes paid for net share settlement of equity awards	$ (106)		(106)
Issuance of common stock in at-the-market offering	67,189	$ 2	67,187
Issuance of common stock in at-the-market offering (shares)		12,313,452
Stock-based compensation	8,568		8,568
Unrealized (losses) gains on available-for-sale securities	7				7
Net loss	(121,931)			(121,931)
Ending balance at Sep. 30, 2021	81,432	$ 11	758,114	(676,696)	3
Ending balance (shares) at Sep. 30, 2021		106,771,584
Beginning balance at Jun. 30, 2021	117,569	$ 10	750,664	(633,109)	4
Beginning balance (shares) at Jun. 30, 2021		102,967,017
Issuance of common stock under employee stock purchase plan	341		341
Issuance of common stock under employee stock purchase plan (shares)		284,456
Issuance of common stock for services	190		190
Issuance of common stock for services (shares)		25,989
Issuance of common stock upon exercise of options	21		21
Issuance of common stock upon exercise of options (shares)		126,004
Issuance of common stock upon vesting of restricted stock units (shares)		34,410
Taxes paid for net share settlement of equity awards	(106)		(106)
Issuance of common stock in at-the-market offering	4,743	$ 1	4,742
Issuance of common stock in at-the-market offering (shares)		3,333,708
Stock-based compensation	2,262		2,262
Unrealized (losses) gains on available-for-sale securities	(1)				(1)
Net loss	(43,587)			(43,587)
Ending balance at Sep. 30, 2021	$ 81,432	$ 11	$ 758,114	$ (676,696)	$ 3
Ending balance (shares) at Sep. 30, 2021		106,771,584