Segments - Summary of Geographic Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	$ 14,209,608	$ 13,071,589	$ 10,855,810
Property and equipment, net	617,739	560,756
UNITED STATES [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	7,424,249	6,917,221	5,991,826
Property and equipment, net	432,102	396,608
United Kingdom [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	2,104,517	2,008,776	1,861,199
Property and equipment, net	61,335	61,327
All other countries [Member]
Revenues From External Customers And Long Lived Assets [Line Items]
Revenue	4,680,842	4,145,592	$ 3,002,785
Property and equipment, net	$ 124,302	$ 102,821