Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets, Net [Abstract]
Intangible assets, accumulated amortization	$ (6,094)	$ (5,495)
Intangible assets, gross carrying amount	19,397	19,006
Intangible assets, net carrying amount	13,303	13,511
Customer contracts and relationships and covenants not to compete
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	11,848	11,485
Intangible assets, accumulated amortization	(5,345)	(4,802)
Net Carrying Amount	6,503	6,683
Favorable leases and other
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	1,151	1,123
Intangible assets, accumulated amortization	(749)	(693)
Net Carrying Amount	402	430
Trademarks (indefinitely-lived)
Intangible assets
Impairment of intangible assets, indefinite-lived	0
Finite-Lived Intangible Assets, Net [Abstract]
Indefinite-lived intangible assets	$ 6,398	$ 6,398