Goodwill and Intangible Assets - Components of Identifiable Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 1,330	$ 1,330
Accumulated Amortization	(1,192)	(1,162)
Net Carrying Amount	138	168
Customer lists and user base
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	613	613
Accumulated Amortization	(574)	(563)
Net Carrying Amount	$ 39	$ 50
Intangible assets acquired, useful life	3 years	3 years
Marketing related
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 198	$ 198
Accumulated Amortization	(197)	(196)
Net Carrying Amount	$ 1	$ 2
Intangible assets acquired, useful life	1 year	1 year
Developed technologies
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 274	$ 274
Accumulated Amortization	(227)	(215)
Net Carrying Amount	$ 47	$ 59
Intangible assets acquired, useful life	3 years	3 years
All other
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 245	$ 245
Accumulated Amortization	(194)	(188)
Net Carrying Amount	$ 51	$ 57
Intangible assets acquired, useful life	5 years	5 years