CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Accumulated Other Comprehensive Income (loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2019		44,864
Beginning balance at Dec. 31, 2019	$ 111,091	$ 489,749	$ 21	$ (378,679)
Increase (Decrease) in Shareholders' Equity
Net loss	(4,705)			(4,705)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	3,768	$ 3,768
Stock option exercises (in shares)		57
Stock option exercises	196	$ 196
Shares issued under the Employee Stock Purchase Plan (in shares)		20
Shares issued under the Employee Stock Purchase Plan	224	$ 224
Issuance of stock upon restricted stock unit vesting (in shares)		36
Issuance of stock upon restricted stock unit vesting	0
Restricted stock withheld for employee tax remittance (in shares)		(14)
Restricted stock withheld for employee tax remittance	(163)	$ (163)
Unrealized gain (loss) on investments	41		41
Ending balance (in shares) at Mar. 31, 2020		44,963
Ending balance at Mar. 31, 2020	110,452	$ 493,774	62	(383,384)
Beginning balance (in shares) at Dec. 31, 2019		44,864
Beginning balance at Dec. 31, 2019	111,091	$ 489,749	21	(378,679)
Increase (Decrease) in Shareholders' Equity
Net loss	(9,356)
Unrealized gain (loss) on investments	57
Ending balance (in shares) at Sep. 30, 2020		45,315
Ending balance at Sep. 30, 2020	114,630	$ 502,587	78	(388,035)
Beginning balance (in shares) at Mar. 31, 2020		44,963
Beginning balance at Mar. 31, 2020	110,452	$ 493,774	62	(383,384)
Increase (Decrease) in Shareholders' Equity
Net loss	(8,269)			(8,269)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	4,376	$ 4,376
Stock option exercises (in shares)		188
Stock option exercises	696	$ 696
Shares issued under the Employee Stock Purchase Plan (in shares)		32
Shares issued under the Employee Stock Purchase Plan	257	$ 257
Issuance of stock upon restricted stock unit vesting (in shares)		11
Issuance of stock upon restricted stock unit vesting	0
Unrealized gain (loss) on investments	84		84
Ending balance (in shares) at Jun. 30, 2020		45,194
Ending balance at Jun. 30, 2020	107,596	$ 499,103	146	(391,653)
Increase (Decrease) in Shareholders' Equity
Net loss	3,618			3,618
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	2,675	$ 2,675
Stock option exercises (in shares)		77
Stock option exercises	500	$ 500
Shares issued under the Employee Stock Purchase Plan (in shares)		44
Shares issued under the Employee Stock Purchase Plan	309	$ 309
Unrealized gain (loss) on investments	(68)		(68)
Ending balance (in shares) at Sep. 30, 2020		45,315
Ending balance at Sep. 30, 2020	114,630	$ 502,587	78	(388,035)
Beginning balance (in shares) at Dec. 31, 2020		45,804
Beginning balance at Dec. 31, 2020	134,260	$ 510,061	14	(375,815)
Increase (Decrease) in Shareholders' Equity
Net loss	(3,289)			(3,289)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	7,019	$ 7,019
Stock option exercises (in shares)		359
Stock option exercises	3,532	$ 3,532
Shares issued under the Employee Stock Purchase Plan (in shares)		14
Shares issued under the Employee Stock Purchase Plan	249	$ 249
Issuance of stock upon restricted stock unit vesting (in shares)		76
Issuance of stock upon restricted stock unit vesting	0
Restricted stock withheld for employee tax remittance (in shares)		(28)
Restricted stock withheld for employee tax remittance	(1,501)	$ (1,501)
Unrealized gain (loss) on investments	(61)		(61)
Ending balance (in shares) at Mar. 31, 2021		46,225
Ending balance at Mar. 31, 2021	140,209	$ 519,360	(47)	(379,104)
Beginning balance (in shares) at Dec. 31, 2020		45,804
Beginning balance at Dec. 31, 2020	134,260	$ 510,061	14	(375,815)
Increase (Decrease) in Shareholders' Equity
Net loss	(12,006)
Unrealized gain (loss) on investments	(37)
Ending balance (in shares) at Sep. 30, 2021		46,767
Ending balance at Sep. 30, 2021	156,780	$ 544,624	(23)	(387,821)
Beginning balance (in shares) at Mar. 31, 2021		46,225
Beginning balance at Mar. 31, 2021	140,209	$ 519,360	(47)	(379,104)
Increase (Decrease) in Shareholders' Equity
Net loss	(3,786)			(3,786)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	10,866	$ 10,866
Stock option exercises (in shares)		330
Stock option exercises	3,531	$ 3,531
Shares issued under the Employee Stock Purchase Plan (in shares)		13
Shares issued under the Employee Stock Purchase Plan	309	$ 309
Issuance of stock upon restricted stock unit vesting (in shares)		12
Issuance of stock upon restricted stock unit vesting	0
Restricted stock withheld for employee tax remittance (in shares)		(1)
Restricted stock withheld for employee tax remittance	(61)	$ (61)
Unrealized gain (loss) on investments	24		24
Ending balance (in shares) at Jun. 30, 2021		46,579
Ending balance at Jun. 30, 2021	151,092	$ 534,005	(23)	(382,890)
Increase (Decrease) in Shareholders' Equity
Net loss	(4,931)			(4,931)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	8,596	$ 8,596
Stock option exercises (in shares)		176
Stock option exercises	1,676	$ 1,676
Shares issued under the Employee Stock Purchase Plan (in shares)		9
Shares issued under the Employee Stock Purchase Plan	404	$ 404
Issuance of stock upon restricted stock unit vesting (in shares)		4
Issuance of stock upon restricted stock unit vesting	0
Restricted stock withheld for employee tax remittance (in shares)		(1)
Restricted stock withheld for employee tax remittance	(57)	$ (57)
Unrealized gain (loss) on investments	0
Ending balance (in shares) at Sep. 30, 2021		46,767
Ending balance at Sep. 30, 2021	$ 156,780	$ 544,624	$ (23)	$ (387,821)