Goodwill, Customer Relationships and Other Intangible Assets - Schedule of Goodwill, Customer Relationships, and Other Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill	$ 30,715	$ 30,475
Indefinite-life intangible assets	269	269
Total other intangible assets, net	1,869	1,897
Customer relationships
Goodwill [Line Items]
Finite-lived intangible assets, net	9,667	10,876
Accumulated amortization	(7,803)	(7,096)
Computer Software, Intangible Asset
Goodwill [Line Items]
Finite-lived intangible assets, net	1,454	1,469
Accumulated amortization	(2,447)	(2,294)
Trade Names and Patents
Goodwill [Line Items]
Finite-lived intangible assets, net	146	159
Accumulated amortization	$ (46)	$ (31)