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 and Intangible Assets Disclosure [Abstract]
Valuation allowance, available to reduce goodwill					$ 16
Acquired finite-lived intangible asset amortization expense	$ 204	$ 49	$ 595	$ 153
Intangible assets, gross, including goodwill	$ 20,100		$ 20,100