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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 51,533	$ 51,336	$ 46,677
Equity in earnings of affiliates	(54)	(43)	(29)
Adjusted segment EBITDA	10,037	9,857	8,949
Depreciation and amortization	2,721	2,596	2,278
Interest expense	1,584	1,824	1,755
Losses (gains) on sales of facilities	7	(18)	(428)
Losses on retirement of debt	295	211	9
Income before income taxes	5,430	5,244	5,335
Assets	47,490	45,058	39,207
Goodwill and other intangible assets, Beginning Balance	8,269	7,953	7,394
Goodwill and other intangible assets, Acquisitions	344	332	636
Goodwill and other intangible assets, Foreign currency translation and other		(16)	(77)
Goodwill and other intangible assets, Foreign currency translation, amortization and other	(35)
Goodwill and other intangible assets, Ending Balance	8,578	8,269	7,953
National Group [Member]
Segment Reporting Information [Line Items]
Revenues	25,694	25,913	22,581
Equity in earnings of affiliates	(28)	(2)	(4)
Adjusted segment EBITDA	5,532	5,634	4,980
Depreciation and amortization	1,216	1,161	946
Assets	18,913	18,290	14,839
Goodwill and other intangible assets, Beginning Balance	1,739	1,597	1,474
Goodwill and other intangible assets, Acquisitions	38	155	132
Goodwill and other intangible assets, Foreign currency translation and other		(13)	(9)
Goodwill and other intangible assets, Foreign currency translation, amortization and other	(2)
Goodwill and other intangible assets, Ending Balance	1,775	1,739	1,597
American Group [Member]
Segment Reporting Information [Line Items]
Revenues	23,593	23,173	21,959
Equity in earnings of affiliates	(42)	(44)	(40)
Adjusted segment EBITDA	5,333	4,904	4,593
Depreciation and amortization	1,164	1,117	1,027
Assets	20,760	20,608	19,122
Goodwill and other intangible assets, Beginning Balance	5,765	5,729	5,265
Goodwill and other intangible assets, Acquisitions	27	39	504
Goodwill and other intangible assets, Foreign currency translation and other		(3)	(40)
Goodwill and other intangible assets, Foreign currency translation, amortization and other	(17)
Goodwill and other intangible assets, Ending Balance	5,775	5,765	5,729
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	2,246	2,250	2,137
Equity in earnings of affiliates	16	3	15
Adjusted segment EBITDA	(828)	(681)	(624)
Depreciation and amortization	341	318	305
Assets	7,817	6,160	5,246
Goodwill and other intangible assets, Beginning Balance	765	627	655
Goodwill and other intangible assets, Acquisitions	279	138
Goodwill and other intangible assets, Foreign currency translation and other			(28)
Goodwill and other intangible assets, Foreign currency translation, amortization and other	(16)
Goodwill and other intangible assets, Ending Balance	$ 1,028	$ 765	$ 627