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 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 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 loss	(22,636)
Unrealized gain on investments	38
Ending balance (in shares) at Jun. 30, 2019		44,066
Ending balance at Jun. 30, 2019	88,542	$ 480,050	104	38	(391,650)
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 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 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)
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 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 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 loss	(12,974)
Unrealized gain on investments	125
Ending balance (in shares) at Jun. 30, 2020		45,194
Ending balance at Jun. 30, 2020	107,596	$ 499,103	0	146	(391,653)
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 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 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)