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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 12,274	$ 11,451	$ 11,529	$ 11,423	$ 11,562	$ 10,696	$ 10,733	$ 10,623	$ 46,677	$ 43,614	$ 41,490
Equity in earnings of affiliates									29	45	54
Adjusted segment EBITDA									8,949	8,233	8,218
Depreciation and amortization									2,278	2,131	1,966
Interest expense									1,755	1,690	1,707
Gains on sales of facilities									(428)	(8)	(23)
Losses on retirement of debt									9	39	4
Legal claim benefits											(246)
Income before income taxes									5,335	4,381	4,810
Assets	39,207				36,593				39,207	36,593	33,758
Goodwill and other intangible assets, Beginning Balance				7,394				6,704	7,394	6,704	6,731
Goodwill and other intangible assets, Acquisitions									636	693	41
Goodwill and other intangible assets, Foreign currency translation and other										(3)	(68)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(77)
Goodwill and other intangible assets, Ending Balance	7,953				7,394				7,953	7,394	6,704
National Group [Member]
Segment Reporting Information [Line Items]
Revenues									22,581	20,772	19,845
Equity in earnings of affiliates									(4)	(21)	(20)
Adjusted segment EBITDA									4,980	4,600	4,565
Depreciation and amortization									946	867	806
Assets	14,839				13,097				14,839	13,097	12,320
Goodwill and other intangible assets, Beginning Balance				1,474				1,458	1,474	1,458	1,481
Goodwill and other intangible assets, Acquisitions									132	19
Goodwill and other intangible assets, Foreign currency translation and other										(3)	(23)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(9)
Goodwill and other intangible assets, Ending Balance	1,597				1,474				1,597	1,474	1,458
American Group [Member]
Segment Reporting Information [Line Items]
Revenues									21,959	20,912	19,670
Equity in earnings of affiliates									(40)	(37)	(38)
Adjusted segment EBITDA									4,593	4,231	4,173
Depreciation and amortization									1,027	986	908
Assets	19,122				18,136				19,122	18,136	16,208
Goodwill and other intangible assets, Beginning Balance				5,265				4,661	5,265	4,661	4,638
Goodwill and other intangible assets, Acquisitions									504	612	33
Goodwill and other intangible assets, Foreign currency translation and other										(8)	(10)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(40)
Goodwill and other intangible assets, Ending Balance	5,729				5,265				5,729	5,265	4,661
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues									2,137	1,930	1,975
Equity in earnings of affiliates									15	13	4
Adjusted segment EBITDA									(624)	(598)	(520)
Depreciation and amortization									305	278	252
Assets	5,246				5,360				5,246	5,360	5,230
Goodwill and other intangible assets, Beginning Balance				$ 655				$ 585	655	585	612
Goodwill and other intangible assets, Acquisitions										62	8
Goodwill and other intangible assets, Foreign currency translation and other										8	(35)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(28)
Goodwill and other intangible assets, Ending Balance	$ 627				$ 655				$ 627	$ 655	$ 585