Goodwill, Customer Relationships and Other Intangible Assets (Details) $ in Millions		12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 9,360	$ 9,354
Gross carrying amounts of goodwill, customer relationships and other intangible assets		17,057
Amortization expense for intangible assets		$ 671	767	$ 871
Number of reporting units	1	1
Estimated amortization expense for intangible assets
2018		$ 583
2019		513
2020		446
2021		136
2022		32
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net		1,362	1,877
Accumulated amortization		4,337	3,822
Capitalized software
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net		379	471
Accumulated amortization		$ 1,619	$ 1,510