Segment and Geographic Information - Schedule of Geographic Distributions of Revenues, Equity in Earnings of Affiliates, Adjusted Segment EBITDA, Depreciation and Amortization and Assets (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 13,523	$ 12,694	$ 12,602	$ 12,517	$ 12,274	$ 11,451	$ 11,529	$ 11,423	$ 51,336	$ 46,677	$ 43,614
Equity in earnings of affiliates									(43)	(29)	(45)
Adjusted segment EBITDA									9,857	8,949	8,233
Depreciation and amortization									2,596	2,278	2,131
Interest expense									1,824	1,755	1,690
Gains on sales of facilities									(18)	(428)	(8)
Losses on retirement of debt									211	9	39
Income before income taxes									5,244	5,335	4,381
Assets	45,058				39,207				45,058	39,207	36,593
Goodwill and other intangible assets, Beginning Balance				7,953				7,394	7,953	7,394	6,704
Goodwill and other intangible assets, Acquisitions									332	636	693
Goodwill and other intangible assets, Foreign currency translation and other										(77)	(3)
Goodwill and other intangible assets, Foreign currency translation, amortization and other									(16)
Goodwill and other intangible assets, Ending Balance	8,269				7,953				8,269	7,953	7,394
National Group [Member]
Segment Reporting Information [Line Items]
Revenues									25,913	22,581	20,772
Equity in earnings of affiliates									(2)	(4)	(21)
Adjusted segment EBITDA									5,634	4,980	4,600
Depreciation and amortization									1,161	946	867
Assets	18,290				14,839				18,290	14,839	13,097
Goodwill and other intangible assets, Beginning Balance				1,597				1,474	1,597	1,474	1,458
Goodwill and other intangible assets, Acquisitions									155	132	19
Goodwill and other intangible assets, Foreign currency translation and other										(9)	(3)
Goodwill and other intangible assets, Foreign currency translation, amortization and other									(13)
Goodwill and other intangible assets, Ending Balance	1,739				1,597				1,739	1,597	1,474
American Group [Member]
Segment Reporting Information [Line Items]
Revenues									23,173	21,959	20,912
Equity in earnings of affiliates									(44)	(40)	(37)
Adjusted segment EBITDA									4,904	4,593	4,231
Depreciation and amortization									1,117	1,027	986
Assets	20,608				19,122				20,608	19,122	18,136
Goodwill and other intangible assets, Beginning Balance				5,729				5,265	5,729	5,265	4,661
Goodwill and other intangible assets, Acquisitions									39	504	612
Goodwill and other intangible assets, Foreign currency translation and other										(40)	(8)
Goodwill and other intangible assets, Foreign currency translation, amortization and other									(3)
Goodwill and other intangible assets, Ending Balance	5,765				5,729				5,765	5,729	5,265
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues									2,250	2,137	1,930
Equity in earnings of affiliates									3	15	13
Adjusted segment EBITDA									(681)	(624)	(598)
Depreciation and amortization									318	305	278
Assets	6,160				5,246				6,160	5,246	5,360
Goodwill and other intangible assets, Beginning Balance				$ 627				$ 655	627	655	585
Goodwill and other intangible assets, Acquisitions									138		62
Goodwill and other intangible assets, Foreign currency translation and other										(28)	8
Goodwill and other intangible assets, Ending Balance	$ 765				$ 627				$ 765	$ 627	$ 655