Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance (shares) at Dec. 31, 2017		48,365,000
Beginning balance at Dec. 31, 2017	$ 24,547	$ 5	$ 340,079	$ (472)	$ (315,065)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (shares)	302,703	303,000
Exercise of stock options	$ 1,858		1,858
Release of stock awards (shares)		824,000
Employee stock-based compensation	4,413		4,413
Taxes paid related to net share settlement of equity awards (in shares)		(297,000)
Taxes paid related to net share settlement of equity awards	(3,140)		(3,140)
Non-employee stock-based compensation	24		24
Issuance of common stock, net of issuance costs (in shares)		4,313,000
Issuance of common stock, net of issuance costs	37,317		37,317
Net loss	(8,429)				(8,429)
Ending balance (shares) at Jun. 30, 2018		53,508,000
Ending balance at Jun. 30, 2018	52,530	$ 5	380,551	0	(328,026)
Beginning balance (shares) at Mar. 31, 2018		48,925,000
Beginning balance at Mar. 31, 2018	15,068	$ 5	339,354	0	(324,291)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (shares)		224,000
Exercise of stock options	1,423		1,423
Release of stock awards (shares)		46,000
Employee stock-based compensation	2,455		2,455
Non-employee stock-based compensation	2		2
Issuance of common stock, net of issuance costs (in shares)		4,313,000
Issuance of common stock, net of issuance costs	37,317		37,317
Net loss	(3,735)				(3,735)
Ending balance (shares) at Jun. 30, 2018		53,508,000
Ending balance at Jun. 30, 2018	52,530	$ 5	380,551	0	(328,026)
Beginning balance (shares) at Dec. 31, 2018		54,065,000
Beginning balance at Dec. 31, 2018	$ 56,306	$ 5	386,775	0	(330,474)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (shares)	529,187	529,000
Exercise of stock options	$ 2,843		2,843
Release of stock awards (shares)		441,000
Employee stock-based compensation	4,051		4,051
Taxes paid related to net share settlement of equity awards (in shares)		(144,000)
Taxes paid related to net share settlement of equity awards	(2,799)		(2,799)
Issuance of common stock, net of issuance costs (in shares)		3,049,000
Issuance of common stock, net of issuance costs	49,926	$ 1	49,925
Net loss	(11,643)				(11,643)
Ending balance (shares) at Jun. 30, 2019		57,940,000
Ending balance at Jun. 30, 2019	98,684	$ 6	440,795	0	(342,117)
Beginning balance (shares) at Mar. 31, 2019		54,541,000
Beginning balance at Mar. 31, 2019	51,210	$ 5	386,815	0	(335,610)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (shares)		310,000
Exercise of stock options	2,067		2,067
Release of stock awards (shares)		40,000
Employee stock-based compensation	1,988		1,988
Issuance of common stock, net of issuance costs (in shares)		3,049,000
Issuance of common stock, net of issuance costs	49,926	$ 1	49,925
Net loss	(6,507)				(6,507)
Ending balance (shares) at Jun. 30, 2019		57,940,000
Ending balance at Jun. 30, 2019	$ 98,684	$ 6	$ 440,795	$ 0	$ (342,117)