Other Financial Data - Components of Fixed Assets, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fixed Assets, net [Line Items]
Fixed assets, gross	$ 1,867	$ 1,780
Less: Accumulated depreciation	1,268	1,159
Fixed assets, net	599	621
Depreciation of fixed assets	167	172	$ 176
Software
Fixed Assets, net [Line Items]
Fixed assets, gross	808	727
Leasehold improvements
Fixed Assets, net [Line Items]
Fixed assets, gross	375	358
Computer equipment
Fixed Assets, net [Line Items]
Fixed assets, gross	249	250
Furniture, fixtures and equipment
Fixed Assets, net [Line Items]
Fixed assets, gross	246	225
Construction in progress
Fixed Assets, net [Line Items]
Fixed assets, gross	155	183
Other
Fixed Assets, net [Line Items]
Fixed assets, gross	$ 34	$ 37