Goodwill, Customer Relationships and Other Intangible Assets - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Goodwill [Line Items]
Intangible assets, net (including goodwill)	$ 19,300		$ 19,300		$ 20,100
Amortization of intangible assets	446	$ 269	1,300	$ 820
Intangible assets, gross (including goodwill)	52,700		52,700
Goodwill	30,770		30,770		30,475
Business
Goodwill [Line Items]
Goodwill	$ 20,492		$ 20,492		$ 20,197