CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Warrants	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2018		43,578
Beginning balance at Dec. 31, 2018	$ 102,231	$ 471,180	$ 104	$ (39)	$ (369,014)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(2,844)				(2,844)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	2,628	$ 2,628
Stock option exercises (in shares)		228
Stock option exercises	780	$ 780
Shares issued under the Employee Stock Purchase Plan (in shares)		19
Shares issued under the Employee Stock Purchase Plan	218	$ 218
Unrealized gain (loss) on investments	42			42
Ending balance (in shares) at Mar. 31, 2019		43,825
Ending balance at Mar. 31, 2019	103,055	$ 474,806	104	3	(371,858)
Beginning balance (in shares) at Dec. 31, 2018		43,578
Beginning balance at Dec. 31, 2018	102,231	$ 471,180	104	(39)	(369,014)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(19,166)
Unrealized gain (loss) on investments	29
Ending balance (in shares) at Sep. 30, 2019		44,520
Ending balance at Sep. 30, 2019	96,990	$ 485,141	0	29	(388,180)
Beginning balance (in shares) at Mar. 31, 2019		43,825
Beginning balance at Mar. 31, 2019	103,055	$ 474,806	104	3	(371,858)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	(19,792)				(19,792)
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	4,183	$ 4,183
Stock option exercises (in shares)		227
Stock option exercises	850	$ 850
Shares issued under the Employee Stock Purchase Plan (in shares)		14
Shares issued under the Employee Stock Purchase Plan	211	$ 211
Unrealized gain (loss) on investments	35			35
Ending balance (in shares) at Jun. 30, 2019		44,066
Ending balance at Jun. 30, 2019	88,542	$ 480,050	104	38	(391,650)
Increase (Decrease) in Shareholders' Equity
Net income (loss)	3,470				3,470
Compensation expense related to stock options and restricted stock units granted, net of forfeitures	3,285	$ 3,285
Stock option exercises (in shares)		416
Stock option exercises	1,427	$ 1,427
Shares issued under the Employee Stock Purchase Plan (in shares)		18
Shares issued under the Employee Stock Purchase Plan	275	$ 275
Unrealized gain (loss) on investments	(9)			(9)
Exercise of warrants resulting in the issuance of common stock (in shares)		20
Exercise of warrants resulting in issuance of common stock	0	$ 104	(104)
Ending balance (in shares) at Sep. 30, 2019		44,520
Ending balance at Sep. 30, 2019	96,990	$ 485,141	0	29	(388,180)
Beginning balance (in shares) at Dec. 31, 2019		44,864
Beginning balance at Dec. 31, 2019	111,091	$ 489,749	0	21	(378,679)
Increase (Decrease) in Shareholders' Equity
Net income (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	0	62	(383,384)
Beginning balance (in shares) at Dec. 31, 2019		44,864
Beginning balance at Dec. 31, 2019	111,091	$ 489,749	0	21	(378,679)
Increase (Decrease) in Shareholders' Equity
Net income (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	0	78	(388,035)
Beginning balance (in shares) at Mar. 31, 2020		44,963
Beginning balance at Mar. 31, 2020	110,452	$ 493,774	0	62	(383,384)
Increase (Decrease) in Shareholders' Equity
Net income (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	0	146	(391,653)
Increase (Decrease) in Shareholders' Equity
Net income (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	$ 0	$ 78	$ (388,035)