Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 94,957		$ 94,957		$ 65,380
Accumulated depreciation and amortization	(26,265)		(26,265)		(14,458)
Total property and equipment, net	68,692		68,692		50,922
Depreciation expense	4,300	$ 2,300	11,800	$ 6,000
Capitalized computer software, amortization	3,100	1,200	8,500	2,800
Computer hardware
Property, Plant and Equipment [Line Items]
Total property and equipment	8,602		8,602		5,667
Furniture and equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	3,038		3,038		2,448
Internal-use software development costs
Property, Plant and Equipment [Line Items]
Total property and equipment	73,679		73,679		48,557
Total property and equipment, net	58,700		58,700		42,100
Capitalized computer software additions	7,000	$ 8,400	25,100	$ 20,400
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 9,638		$ 9,638		$ 8,708