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	178		$ 0	178	0	0
Common stock issued upon stock option exercise, Shares			17,000
Common stock issued upon vesting of restricted stock units, net of tax withholding	(3,218)		$ 0	(3,218)	0	0
Common stock issued upon vesting of restricted stock units, net of tax withholding, Shares			335,000
Stock-based compensation expense	18,048		$ 0	18,048	0	0
Comprehensive income (loss)	(262)	[1]	0	0	(262)	0
Net loss	(27,180)		0	0	0	(27,180)
Ending balance at Jun. 30, 2018	27,609		$ 1	232,559	(243)	(204,708)
Ending balance, Shares at Jun. 30, 2018			101,735,000
Beginning Balance at Mar. 31, 2018	32,423		$ 1	224,655	12	(192,245)
Beginning balance, Shares at Mar. 31, 2018			101,564,000
Common stock issued upon stock option exercise	177		$ 0	177	0	0
Common stock issued upon stock option exercise, Shares			17,000
Common stock issued upon vesting of restricted stock units, net of tax withholding	(1,453)		$ 0	(1,453)	0	0
Common stock issued upon vesting of restricted stock units, net of tax withholding, Shares			154,000
Stock-based compensation expense	9,180		$ 0	9,180	0	0
Comprehensive income (loss)	(255)	[1]	0	0	(255)	0
Net loss	(12,463)		0	0	0	(12,463)
Ending balance at Jun. 30, 2018	27,609		$ 1	232,559	(243)	(204,708)
Ending balance, Shares at Jun. 30, 2018			101,735,000
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	$ 37,638		$ 0	37,638	0	0
Common stock issued upon stock option exercise, Shares	2,956,897		2,957,000
Common stock issued upon vesting of restricted stock units	$ 0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			1,780,000
Common stock issued in connection with acquisition	30,092		$ 0	30,092	0	0
Common stock issued in connection with acquisition, shares			1,944,000
Common stock issued under employee stock purchase plan	2,662		$ 0	2,662	0	0
Common stock issued under employee stock purchase plan, Shares			261,000
Stock-based compensation expense	32,296		$ 0	32,296	0	0
Comprehensive income (loss)	788	[1]	0	0	788	0
Other	232		$ 0	232	0	0
Other, Shares			16,000
Net loss	(36,255)		$ 0	0	0	(36,255)
Ending balance at Jun. 30, 2019	289,529		$ 1	654,857	501	(365,830)
Ending balance, Shares at Jun. 30, 2019			132,776,000
Beginning Balance at Mar. 31, 2019	234,995		$ 1	582,652	(306)	(347,352)
Beginning balance, Shares at Mar. 31, 2019			128,060,000
Common stock issued upon stock option exercise	23,219		$ 0	23,219	0	0
Common stock issued upon stock option exercise, Shares			1,629,000
Common stock issued upon vesting of restricted stock units	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, Shares			882,000
Common stock issued in connection with acquisition	30,092		$ 0	30,092	0	0
Common stock issued in connection with acquisition, shares			1,944,000
Common stock issued under employee stock purchase plan	2,662		$ 0	2,662	0	0
Common stock issued under employee stock purchase plan, Shares			261,000
Stock-based compensation expense	16,232		$ 0	16,232	0	0
Comprehensive income (loss)	807	[1]	0	0	807	0
Net loss	(18,478)		0	0	0	(18,478)
Ending balance at Jun. 30, 2019	289,529		$ 1	654,857	501	(365,830)
Ending balance, Shares at Jun. 30, 2019			132,776,000
Cumulative-effect adjustment upon adoption | ASC 842	$ 2,693		$ 0	$ 0	$ 0	$ 2,693

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