Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets
Intangible assets, accumulated amortization	$ (6,864)	$ (6,299)
Intangible assets, gross carrying amount	20,030	19,929
Intangible assets, net carrying amount	13,166	13,630
Customer contracts and relationships and covenants not to compete
Finite-Lived Intangible Assets
Gross Carrying Amount	12,507	12,341
Intangible assets, accumulated amortization	(6,059)	(5,536)
Net Carrying Amount	6,448	6,805
Favorable leases and other
Finite-Lived Intangible Assets
Gross Carrying Amount	1,125	1,190
Intangible assets, accumulated amortization	(805)	(763)
Net Carrying Amount	320	427
Trademarks (indefinitely-lived)
Finite-Lived Intangible Assets
Indefinite-lived intangible assets	$ 6,398	$ 6,398