Goodwill, Customer Relationships and Other Intangible Assets (Details) $ in Millions		12 Months Ended
	Oct. 31, 2017 reporting_unit	Dec. 31, 2018 USD ($) reporting_unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 9,360	$ 9,360
Gross carrying amounts of goodwill, customer relationships and other intangible assets		17,082
Amortization expense for intangible assets		581	671	$ 767
Estimated amortization expense for intangible assets
2019		517
2020		453
2021		143
2022		39
2023		$ 27
Number of reporting units | reporting_unit	1	1
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net		$ 893	1,362
Accumulated amortization		4,806	4,337
Capitalized software
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net		311	379
Accumulated amortization		$ 1,712	$ 1,619