Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	AOCI Attributable to Parent	Retained Earnings
Balances at Dec. 31, 2019	$ 60,081	$ 3	$ 255,142	$ 51	$ (195,115)
Balances (in shares) at Dec. 31, 2019		29,084
Issuance of common stock in connection with employee stock purchase plan	174		174
Issuance of common stock in connection with employee stock purchase plan (in shares)		26
Issuance of common stock in connection with exercise of stock options	115		115
Issuance of common stock in connection with exercise of stock options (in shares)		37
Stock-based compensation expense	1,348		1,348
Unrealized gain (loss) on marketable securities	(185)			(185)
Net loss	(18,728)				(18,728)
Balances at Mar. 31, 2020	42,805	$ 3	256,779	(134)	(213,843)
Balances (in shares) at Mar. 31, 2020		29,147
Balances at Dec. 31, 2019	60,081	$ 3	255,142	51	(195,115)
Balances (in shares) at Dec. 31, 2019		29,084
Foreign currency translation adjustment	5
Unrealized gain (loss) on marketable securities	(36)
Net loss	(58,179)
Balances at Sep. 30, 2020	136,274	$ 5	389,543	20	(253,294)
Balances (in shares) at Sep. 30, 2020		44,692
Balances at Mar. 31, 2020	42,805	$ 3	256,779	(134)	(213,843)
Balances (in shares) at Mar. 31, 2020		29,147
Issuance of common stock in connection with exercise of stock options	44		44
Issuance of common stock in connection with exercise of stock options (in shares)		10
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs	129,054	$ 2	129,052
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs (in shares)		15,443
Stock-based compensation expense	1,600		1,600
Unrealized gain (loss) on marketable securities	173			173
Net loss	(21,418)				(21,418)
Balances at Jun. 30, 2020	152,258	$ 5	387,475	39	(235,261)
Balances (in shares) at Jun. 30, 2020		44,600
Offering costs in connection with public offering	(39)		(39)
Issuance of common stock in connection with employee stock purchase plan	192		192
Issuance of common stock in connection with employee stock purchase plan (in shares)		34
Issuance of common stock in connection with exercise of stock options	219		219
Issuance of common stock in connection with exercise of stock options (in shares)		58
Stock-based compensation expense	1,696		1,696
Foreign currency translation adjustment	5			5
Unrealized gain (loss) on marketable securities	(24)			(24)
Net loss	(18,033)				(18,033)
Balances at Sep. 30, 2020	136,274	$ 5	389,543	20	(253,294)
Balances (in shares) at Sep. 30, 2020		44,692
Balances at Dec. 31, 2020	139,832	$ 5	415,824	11	(276,008)
Balances (in shares) at Dec. 31, 2020		47,959
Issuance of common stock in connection with employee stock purchase plan	224		224
Issuance of common stock in connection with employee stock purchase plan (in shares)		38
Issuance of common stock in connection with exercise of stock options	146		146
Issuance of common stock in connection with exercise of stock options (in shares)		23
Issuance of common stock and pre-funded warrants in connection with public offering, net of offering costs (in shares)		1,000
Stock-based compensation expense	1,757		1,757
Foreign currency translation adjustment	4			4
Unrealized gain (loss) on marketable securities	10			10
Net loss	(18,218)				(18,218)
Balances at Mar. 31, 2021	123,755	$ 5	417,951	25	(294,226)
Balances (in shares) at Mar. 31, 2021		49,020
Balances at Dec. 31, 2020	139,832	$ 5	415,824	11	(276,008)
Balances (in shares) at Dec. 31, 2020		47,959
Foreign currency translation adjustment	(4)
Unrealized gain (loss) on marketable securities	(5)
Net loss	(45,355)
Balances at Sep. 30, 2021	102,081	$ 5	423,437	2	(321,363)
Balances (in shares) at Sep. 30, 2021		49,306
Balances at Mar. 31, 2021	123,755	$ 5	417,951	25	(294,226)
Balances (in shares) at Mar. 31, 2021		49,020
Issuance of common stock in connection with exercise of stock options	33		33
Issuance of common stock in connection with exercise of stock options (in shares)		6
Stock-based compensation expense	2,080		2,080
Foreign currency translation adjustment	2			2
Unrealized gain (loss) on marketable securities	(16)			(16)
Net loss	(6,830)				(6,830)
Balances at Jun. 30, 2021	119,024	$ 5	420,064	11	(301,056)
Balances (in shares) at Jun. 30, 2021		49,026
Issuance of common stock in connection with employee stock purchase plan	230		230
Issuance of common stock in connection with employee stock purchase plan (in shares)		45
Issuance of common stock in connection with exercise of stock options	1,036		1,036
Issuance of common stock in connection with exercise of stock options (in shares)		235
Stock-based compensation expense	2,107		2,107
Foreign currency translation adjustment	(10)			(10)
Unrealized gain (loss) on marketable securities	1			1
Net loss	(20,307)				(20,307)
Balances at Sep. 30, 2021	$ 102,081	$ 5	$ 423,437	$ 2	$ (321,363)
Balances (in shares) at Sep. 30, 2021		49,306