CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (unaudited) - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2017	$ 40,043		$ 1	$ 217,594	$ 19	$ (177,571)
Beginning balance, Shares at Dec. 31, 2017			101,383,000
Common stock issued upon stock option exercise	1		$ 0	1	0	0
Common stock issued upon stock option exercise, Shares			0
Common stock issued upon vesting of restricted stock units, net of tax withholding	(1,765)		$ 0	(1,765)	0	0
Common stock issued upon vesting of restricted stock units, net of tax withholding, Shares			181,000
Stock-based compensation expense	8,868		$ 0	8,868	0	0
Comprehensive loss	(7)	[1]	0	0	(7)	0
Net loss	(14,717)		0	0	0	(14,717)
Ending balance at Mar. 31, 2018	32,423		$ 1	224,655	12	(192,245)
Ending balance, Shares at Mar. 31, 2018			101,564,000
Cumulative-effect adjustment upon adoption | Accounting Standards Update 2017-09	0		$ 0	(43)	0	43
Beginning Balance at Dec. 31, 2018	219,383		$ 1	551,937	(287)	(332,268)
Beginning balance, Shares at Dec. 31, 2018			125,818,000
Common stock issued upon stock option exercise	$ 14,419		$ 0	14,419	0	0
Common stock issued upon stock option exercise, Shares	1,327,784		1,328,000
Common stock issued upon vesting of restricted stock units, Shares			898,000
Stock-based compensation expense	$ 16,064		$ 0	16,064	0	0
Comprehensive loss	(19)	[1]	0	0	(19)	0
Other	232		$ 0	232	0	0
Other, Shares			16,000
Net loss	(17,777)		$ 0	0	0	(17,777)
Ending balance at Mar. 31, 2019	234,995		$ 1	582,652	(306)	(347,352)
Ending balance, Shares at Mar. 31, 2019			128,060,000
Cumulative-effect adjustment upon adoption | ASC 842	$ 2,693		$ 0	$ 0	$ 0	$ 2,693

[1] Net of tax effect which was not material.