Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 897,141	$ 545,267
Less accumulated depreciation and amortization	(290,164)	(184,126)
Property and equipment, net	606,977	361,141
Depreciation and amortization	122,600	85,900	$ 54,600
Furniture and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	339,761	213,790
Site and software development costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	172,653	118,356
Property and equipment, net	62,900	45,400
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	109,929	82,614
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	90,104	46,826
Building
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 184,694	$ 83,681