CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2015	$ 79,186		$ 1	$ 159,180	$ (1,397)	$ (78,598)
Begining balance, Shares at Dec. 31, 2015			98,874,000
Common stock issued upon stock option exercise	$ 201		$ 0	201	0	0
Common stock issued upon stock option exercise, Shares	45,444		46,000
Common stock issued upon vesting of restricted stock units, net of tax withholding	$ (11,934)		$ 0	(11,934)	0	0
Common stock issued upon vesting of restricted stock units, net of tax withholding, Shares			1,566,000
Stock-based compensation expense	39,047		$ 0	39,047	0	0
Comprehensive income (loss)	1,484	[1]	0	0	1,484	0
Net loss	(76,350)		0	0	0	(76,350)
Ending balance at Dec. 31, 2016	33,021		$ 1	186,494	87	(153,561)
Ending balance, Shares at Dec. 31, 2016			100,486,000
Cumulative-effect adjustment upon adoption | ASC 606	1,387		$ 0	0	0	1,387
Common stock issued upon stock option exercise	$ 153		$ 0	153	0	0
Common stock issued upon stock option exercise, Shares	35,352		35,000
Common stock issued upon vesting of restricted stock units, net of tax withholding	$ (6,934)		$ 0	(6,934)	0	0
Common stock issued upon vesting of restricted stock units, net of tax withholding, Shares			879,000
Repurchase of common stock	(144)		$ 0	(144)	0	0
Repurchase of common stock, Shares			(11,791)
Forfeiture of restricted stock awards	(80)		$ 0	(80)	0	0
Forfeiture of restricted stock awards, Shares			(5,000)
Stock-based compensation expense	38,105		$ 0	38,105	0	0
Comprehensive income (loss)	(68)	[1]	0	0	(68)	0
Net loss	(24,010)		0	0	0	(24,010)
Ending balance at Dec. 31, 2017	40,043		$ 1	217,594	19	(177,571)
Ending balance, Shares at Dec. 31, 2017			101,383,000
Issuance of common stock in connection with initial public offering and concurrent private placement, net of underwriting discounts and issuance costs	225,336		$ 0	225,336	0	0
Issuance of common stock in connection with initial public offering and concurrent private placement, net of underwriting discounts and issuance costs, Shares			20,583,000
Common stock issued upon stock option exercise	$ 494		$ 0	494	0	0
Common stock issued upon stock option exercise, Shares	81,849		82,000
Common stock issued upon vesting of restricted stock units, net of tax withholding	$ (25,807)		$ 0	(25,807)	0	0
Common stock issued upon vesting of restricted stock units, net of tax withholding, Shares			3,771,000
Repurchase of common stock	(16)		$ 0	(16)	0	0
Repurchase of common stock, Shares			(1,159)
Stock-based compensation expense	134,379		$ 0	134,379	0	0
Comprehensive income (loss)	(306)	[1]	0	0	(306)	0
Net loss	(154,740)		0	0	0	(154,740)
Ending balance at Dec. 31, 2018	219,383		$ 1	551,937	(287)	(332,268)
Ending balance, Shares at Dec. 31, 2018			125,818,000
Cumulative-effect adjustment upon adoption | ASU 2017-09	$ 0		$ 0	$ (43)	$ 0	$ 43

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