Goodwill, Customer Relationships and Other Intangible Assets - Goodwill Activity (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 11,119	$ 7,415	$ 11,119
Goodwill Impairment	$ (3,700)	0	(3,708)	$ 0
Effect of foreign currency exchange rate changes and other		(10)	4
Goodwill, ending balance		7,405	7,415	$ 11,119
Goodwill, accumulated impairment loss		$ 3,700	$ 3,700