Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Amortization and depreciation expense related to property and equipment other than website development costs	$ 15,600	$ 13,500	$ 12,200
Capitalization of website development costs	49,900	49,500	46,100
Amortization of website development costs and intangible assets included in technology and development	94,349	87,060	63,189
Technology and Development
Property, Plant and Equipment [Line Items]
Amortization of website development costs and intangible assets included in technology and development	54,300	47,000	39,300
Technology and Development | Software Development
Property, Plant and Equipment [Line Items]
Amortization of website development costs and intangible assets included in technology and development	$ 40,000	$ 40,000	$ 23,900