Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balances at Dec. 31, 2017	$ 26,944	$ 3	$ 250,899	$ (1)	$ (223,957)
Balances, Shares at Dec. 31, 2017		30,860
Exercise of common stock options	5,731		5,731
Exercise of common stock options, Shares		771
Vesting of restricted stock units, net, Shares		449
Purchase of ESPP shares	3,030		3,030
Purchase of ESPP shares, Shares		105
Stock-based compensation	16,674		16,674
Common stock and options issued in acquisition, Shares		2
Secondary offering	109,789		109,789
Secondary offering, Shares		2,875
Unrealized gain (loss) on marketable securities	(11)			(11)
Shares withheld for tax withholding on vesting of restricted stock	(334)		(334)
Shares withheld for tax withholding on vesting of restricted stock, Shares		(8)
Net loss	(35,878)				(35,878)
Balances at Sep. 30, 2018	125,945	$ 3	385,789	(12)	(259,835)
Balances, Shares at Sep. 30, 2018		35,054
Balances at Jun. 30, 2018	130,123	$ 3	378,485	(2)	(248,363)
Balances, Shares at Jun. 30, 2018		34,765
Exercise of common stock options	1,513		1,513
Exercise of common stock options, Shares		151
Vesting of restricted stock units, net, Shares		139
Stock-based compensation	5,871		5,871
Common stock and options issued in acquisition, Shares		2
Unrealized gain (loss) on marketable securities	(10)			(10)
Shares withheld for tax withholding on vesting of restricted stock	(80)		(80)
Shares withheld for tax withholding on vesting of restricted stock, Shares		(3)
Net loss	(11,472)				(11,472)
Balances at Sep. 30, 2018	125,945	$ 3	385,789	(12)	(259,835)
Balances, Shares at Sep. 30, 2018		35,054
Balances at Dec. 31, 2018	128,438	$ 3	395,865	(8)	(267,422)
Balances, Shares at Dec. 31, 2018		35,386
Exercise of common stock options	$ 3,943		3,943
Exercise of common stock options, Shares	436	436
Vesting of restricted stock units, net, Shares		1,025
Purchase of ESPP shares	$ 3,066		3,066
Purchase of ESPP shares, Shares		95
Stock-based compensation	45,161		45,161
Common stock and options issued in acquisition	30,012		30,012
Common stock and options issued in acquisition, Shares		666
Unrealized gain (loss) on marketable securities	18			18
Shares withheld for tax withholding on vesting of restricted stock	(2,094)		(2,094)
Shares withheld for tax withholding on vesting of restricted stock, Shares		(49)
Net loss	(57,814)				(57,814)
Balances at Sep. 30, 2019	150,730	$ 3	475,953	10	(325,236)
Balances, Shares at Sep. 30, 2019		37,559
Balances at Jun. 30, 2019	156,548	$ 3	460,841	17	(304,313)
Balances, Shares at Jun. 30, 2019		37,062
Exercise of common stock options	1,446		1,446
Exercise of common stock options, Shares		141
Vesting of restricted stock units, net, Shares		391
Stock-based compensation	15,122		15,122
Unrealized gain (loss) on marketable securities	(7)			(7)
Shares withheld for tax withholding on vesting of restricted stock	(1,456)		(1,456)
Shares withheld for tax withholding on vesting of restricted stock, Shares		(35)
Net loss	(20,923)				(20,923)
Balances at Sep. 30, 2019	$ 150,730	$ 3	$ 475,953	$ 10	$ (325,236)
Balances, Shares at Sep. 30, 2019		37,559