Property and Equipment, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Cost of property and equipment	$ 979	$ 805
Less accumulated depreciation	(685)	(547)
Property and equipment, net	294	258	$ 189
Depreciation expense	130	121	82
Rental expense	71	65	$ 39
Land
Property, Plant and Equipment [Line Items]
Cost of property and equipment	1	1
Buildings
Property, Plant and Equipment [Line Items]
Cost of property and equipment	4	4
Leasehold improvements
Property, Plant and Equipment [Line Items]
Cost of property and equipment	224	162
Computer equipment
Property, Plant and Equipment [Line Items]
Cost of property and equipment	658	560
Office furniture and other equipment
Property, Plant and Equipment [Line Items]
Cost of property and equipment	$ 92	$ 78