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	440		$ 0	440	0	0
Common stock issued upon stock option exercise, Shares			66,000
Common stock issued upon vesting of restricted stock units, net	(25,803)		$ 0	(25,803)	0	0
Common stock issued upon vesting of restricted stock units, net, Shares			3,046,000
Issuance of common stock in connection with initial public offering, net	225,336		$ 0	225,336	0	0
Issuance of common stock in connection with initial public offering, net, Shares			20,583,000
Repurchase of common stock	(16)		$ 0	(16)	0	0
Repurchase of common stock, Shares			(1,000)
Stock-based compensation expense	117,355		$ 0	117,355	0	0
Comprehensive loss	(334)	[1]	0	0	(334)	0
Net loss	(129,542)		0	0	0	(129,542)
Ending balance at Sep. 30, 2018	227,479		$ 1	534,863	(315)	(307,070)
Ending balance, Shares at Sep. 30, 2018			125,077,000
Beginning Balance at Jun. 30, 2018	27,609		$ 1	232,559	(243)	(204,708)
Beginning balance, Shares at Jun. 30, 2018			101,735,000
Common stock issued upon stock option exercise	262		$ 0	262	0	0
Common stock issued upon stock option exercise, Shares			49,000
Common stock issued upon vesting of restricted stock units, net	(22,585)		$ 0	(22,585)	0	0
Common stock issued upon vesting of restricted stock units, net, Shares			2,711,000
Issuance of common stock in connection with initial public offering, net	225,336		$ 0	225,336	0	0
Issuance of common stock in connection with initial public offering, net, Shares			20,583,000
Repurchase of common stock	(16)		$ 0	(16)	0	0
Repurchase of common stock, Shares			(1,000)
Stock-based compensation expense	99,307		$ 0	99,307	0	0
Comprehensive loss	(72)	[1]	0	0	(72)	0
Net loss	(102,362)		0	0	0	(102,362)
Ending balance at Sep. 30, 2018	227,479		$ 1	534,863	(315)	(307,070)
Ending balance, Shares at Sep. 30, 2018			125,077,000
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	$ 41,986		$ 0	41,986	0	0
Common stock issued upon stock option exercise, Shares	3,319,196		3,319,000
Common stock issued upon vesting of restricted stock units, net	$ 0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, net, Shares			2,860,000
Common stock issued in connection with acquisitions	36,204		$ 0	36,204	0	0
Common stock issued in connection with acquisitions, shares			2,320,000
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	47,936		$ 0	47,936	0	0
Comprehensive loss	(2,162)	[1]	0	0	(2,162)	0
Other	232		$ 0	232	0	0
Other, Shares			16,000
Net loss	(52,565)		$ 0	0	0	(52,565)
Ending balance at Sep. 30, 2019	296,369		$ 1	680,957	(2,449)	(382,140)
Ending balance, Shares at Sep. 30, 2019			134,594,000
Beginning Balance at Jun. 30, 2019	289,529		$ 1	654,857	501	(365,830)
Beginning balance, Shares at Jun. 30, 2019			132,776,000
Common stock issued upon stock option exercise	4,348		$ 0	4,348	0	0
Common stock issued upon stock option exercise, Shares			362,000
Common stock issued upon vesting of restricted stock units, net	0		$ 0	0	0	0
Common stock issued upon vesting of restricted stock units, net, Shares			1,080,000
Common stock issued in connection with acquisitions	6,112		$ 0	6,112	0	0
Common stock issued in connection with acquisitions, shares			376,000
Stock-based compensation expense	15,640		$ 0	15,640	0	0
Comprehensive loss	(2,950)	[1]	0	0	(2,950)	0
Net loss	(16,310)		0	0	0	(16,310)
Ending balance at Sep. 30, 2019	296,369		$ 1	680,957	(2,449)	(382,140)
Ending balance, Shares at Sep. 30, 2019			134,594,000
Cumulative-effect adjustment upon adoption | ASC 842	$ 2,693		$ 0	$ 0	$ 0	$ 2,693

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