CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Warrants Amounts	Accumulated Other Comprehensive Income	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		35,861,000
Beginning balance at Dec. 31, 2017	$ 22,540	$ 383,020	$ 397	$ 0	$ (360,877)
Increase (Decrease) in Shareholders' Equity
Net loss	(7,659)				(7,659)
Compensation expense related to stock options granted, net of forfeitures	1,348	$ 1,348
Stock option exercises (in shares)		253,000
Stock option exercises	851	$ 851
Shares issued under the Employee Stock Purchase Plan ( in shares )		28,000
Shares issued under the Employee Stock Purchase Plan	127	$ 127
Exercise of warrants resulting in the issuance of common stock (Note 11) (in shares)		360,000
Exercise of warrants resulting in the issuance of common stock	1,727	$ 1,727
Net change in warrant valuation of exercised warrants	2,001	$ 2,001
Ending balance (in shares) at Mar. 31, 2018		36,502,000
Ending balance at Mar. 31, 2018	20,935	$ 389,074	397	0	(368,536)
Beginning balance (in shares) at Dec. 31, 2017		35,861,000
Beginning balance at Dec. 31, 2017	22,540	$ 383,020	397	0	(360,877)
Increase (Decrease) in Shareholders' Equity
Net loss	(12,310)
Ending balance (in shares) at Jun. 30, 2018		42,684,000
Ending balance at Jun. 30, 2018	90,598	$ 463,483	302	0	(373,187)
Beginning balance (in shares) at Mar. 31, 2018		36,502,000
Beginning balance at Mar. 31, 2018	20,935	$ 389,074	397	0	(368,536)
Increase (Decrease) in Shareholders' Equity
Net loss	(4,651)				(4,651)
Compensation expense related to stock options granted, net of forfeitures	2,465	$ 2,465
Issuance of common stock, net of issuance costs (in shares)		5,750,000
Issuance of common stock, net of issuance costs	70,090	$ 70,090
Stock option exercises (in shares)		306,000
Stock option exercises	964	$ 964
Shares issued under the Employee Stock Purchase Plan ( in shares )		31,000
Shares issued under the Employee Stock Purchase Plan	148	$ 148
Exercise of warrants resulting in the issuance of common stock (Note 11) (in shares)		95,000
Exercise of warrants resulting in the issuance of common stock	238	$ 333	(95)
Net change in warrant valuation of exercised warrants	409	$ 409
Ending balance (in shares) at Jun. 30, 2018		42,684,000
Ending balance at Jun. 30, 2018	90,598	$ 463,483	302	0	(373,187)
Beginning balance (in shares) at Dec. 31, 2018		43,578,000
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 granted, net of forfeitures	2,628	$ 2,628
Stock option exercises (in shares)		228,000
Stock option exercises	780	$ 780
Shares issued under the Employee Stock Purchase Plan ( in shares )		19,000
Shares issued under the Employee Stock Purchase Plan	218	$ 218
Change in unrealized gain (loss) on investments	42			42
Ending balance (in shares) at Mar. 31, 2019		43,825,000
Ending balance at Mar. 31, 2019	103,055	$ 474,806	104	3	(371,858)
Beginning balance (in shares) at Dec. 31, 2018		43,578,000
Beginning balance at Dec. 31, 2018	102,231	$ 471,180	104	(39)	(369,014)
Increase (Decrease) in Shareholders' Equity
Net loss	(22,636)
Ending balance (in shares) at Jun. 30, 2019		44,066,000
Ending balance at Jun. 30, 2019	88,542	$ 480,050	104	38	(391,650)
Beginning balance (in shares) at Mar. 31, 2019		43,825,000
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 granted, net of forfeitures	4,183	$ 4,183
Stock option exercises (in shares)		227,000
Stock option exercises	850	$ 850
Shares issued under the Employee Stock Purchase Plan ( in shares )		14,000
Shares issued under the Employee Stock Purchase Plan	211	$ 211
Change in unrealized gain (loss) on investments	35			35
Ending balance (in shares) at Jun. 30, 2019		44,066,000
Ending balance at Jun. 30, 2019	$ 88,542	$ 480,050	$ 104	$ 38	$ (391,650)