CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Warrants Amounts	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		35,861
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
Stock option exercises	851	$ 851
Shares issued under the Employee Stock Purchase Plan ( in shares )		28
Shares issued under the Employee Stock Purchase Plan	127	$ 127
Exercise of warrants resulting in the issuance of common stock (in shares)		360
Exercise of warrants resulting in 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
Ending balance at Mar. 31, 2018	20,935	$ 389,074	397	0	(368,536)
Beginning balance (in shares) at Dec. 31, 2017		35,861
Beginning balance at Dec. 31, 2017	22,540	$ 383,020	397	0	(360,877)
Increase (Decrease) in Shareholders' Equity
Net loss	(13,379)
Ending balance (in shares) at Sep. 30, 2018		43,170
Ending balance at Sep. 30, 2018	94,475	$ 468,447	302	(18)	(374,256)
Beginning balance (in shares) at Mar. 31, 2018		36,502
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
Issuance of common stock, net of issuance costs	70,090	$ 70,090
Stock option exercises (in shares)		306
Stock option exercises	964	$ 964
Shares issued under the Employee Stock Purchase Plan ( in shares )		31
Shares issued under the Employee Stock Purchase Plan	148	$ 148
Exercise of warrants resulting in the issuance of common stock (in shares)		95
Exercise of warrants resulting in 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
Ending balance at Jun. 30, 2018	90,598	$ 463,483	302	0	(373,187)
Increase (Decrease) in Shareholders' Equity
Net loss	(1,069)				(1,069)
Compensation expense related to stock options granted, net of forfeitures	1,932	$ 1,932
Stock option exercises (in shares)		305
Stock option exercises	952	$ 952
Shares issued under the Employee Stock Purchase Plan ( in shares )		25
Shares issued under the Employee Stock Purchase Plan	200	$ 200
Change in unrealized gain (loss) on investments	(18)			(18)
Exercise of warrants resulting in the issuance of common stock (in shares)		156
Exercise of warrants resulting in issuance of common stock	751	$ 751
Net change in warrant valuation of exercised warrants	(1,129)	$ (1,129)
Ending balance (in shares) at Sep. 30, 2018		43,170
Ending balance at Sep. 30, 2018	94,475	$ 468,447	302	(18)	(374,256)
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 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
Change in 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 loss	(19,166)
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 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
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
Change in 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 loss	3,470				3,470
Compensation expense related to stock options 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
Change in 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		$ 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)