Goodwill, Customer Relationships and Other Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 28,031	$ 30,475	$ 19,650
Finite-lived intangible assets, net	8,911	10,876
Indefinite-life intangible assets	269	269
Other intangible assets, net	1,868	1,897
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	8,911	10,876
Accumulated amortization	8,492	7,096
Capitalized software
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	1,468	1,469
Accumulated amortization	2,616	2,294
Tradenames and patents
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	131	159
Accumulated amortization	$ 61	$ 31