Goodwill, Customer Relationships and Other Intangible Assets - Schedule of Goodwill (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 9,360	$ 9,360
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	1,006	1,362
Accumulated amortization	4,693	4,337
Other Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	328	379
Accumulated amortization	$ 1,684	$ 1,619