Goodwill and Intangible Assets - Components of Identifiable Intangible Assets (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended
	May 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 1,934	$ 1,330
Accumulated Amortization		(1,250)	(1,162)
Net Carrying Amount		684	168
Customer lists and user base
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		964	613
Accumulated Amortization		(598)	(563)
Net Carrying Amount		$ 366	$ 50
Intangible assets acquired, useful life		7 years	3 years
Marketing related
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 300	$ 198
Accumulated Amortization		(199)	(196)
Net Carrying Amount		$ 101	$ 2
Intangible assets acquired, useful life		3 years	1 year
Developed technologies
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 425	$ 274
Accumulated Amortization		(249)	(215)
Net Carrying Amount		$ 176	$ 59
Intangible assets acquired, useful life	2 years	3 years	3 years
All other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 245	$ 245
Accumulated Amortization		(204)	(188)
Net Carrying Amount		$ 41	$ 57
Intangible assets acquired, useful life		5 years	5 years