Consolidated Statements Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (shares) at Dec. 31, 2015		40,343
Beginning balance at Dec. 31, 2015	$ 22,879	$ 4	$ 305,981	$ 405	$ (283,511)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (shares)	398	398
Exercise of stock options	$ 1,034		1,034
Release of stock awards (shares)		911
Employee stock-based compensation	5,673		5,673
Taxes paid related to net share settlement of equity awards (shares)		(397)
Taxes paid related to net share settlement of equity awards	(1,524)		(1,524)
Total comprehensive loss	(8,963)			(405)	(8,558)
Net loss	(8,558)
Ending balance (shares) at Dec. 31, 2016		41,255
Ending balance at Dec. 31, 2016	$ 19,099	$ 4	311,164	0	(292,069)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (shares)	86	86
Exercise of stock options	$ 266		266
Release of stock awards (shares)		1,096
Employee stock-based compensation	7,048		7,048
Non-employee stock-based compensation	43		43
Taxes paid related to net share settlement of equity awards (shares)		(397)
Taxes paid related to net share settlement of equity awards	(1,670)		(1,670)
Issuance of common stock, net of issuance costs (shares)		6,325
Issuance of common stock, net of issuance costs	23,229	$ 1	23,228
Total comprehensive loss	(23,468)			(472)	(22,996)
Net loss	(22,996)
Ending balance (shares) at Dec. 31, 2017		48,365
Ending 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)	772	856
Exercise of stock options	$ 4,680		4,680
Release of stock awards (shares)		832
Employee stock-based compensation	7,865		7,865
Non-employee stock-based compensation	24		24
Taxes paid related to net share settlement of equity awards (shares)		(301)
Taxes paid related to net share settlement of equity awards	(3,190)		(3,190)
Issuance of common stock, net of issuance costs (shares)		4,313
Issuance of common stock, net of issuance costs	37,317		37,317
Total comprehensive loss	(10,878)
Net loss	(10,878)				(10,878)
Ending balance (shares) at Dec. 31, 2018		54,065
Ending balance at Dec. 31, 2018	56,306	$ 5	$ 386,775	0	(330,474)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principles	$ (4,059)			$ 472	$ (4,531)