CONSOLIDATED STATEMENTS OF SHAREHOLDERS EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Warrants	Accumulated Other Comprehensive Income	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, 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)