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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 11,562	$ 10,696	$ 10,733	$ 10,623	$ 10,641	$ 10,270	$ 10,319	$ 10,260	$ 43,614	$ 41,490	$ 39,678
Equity in earnings of affiliates									45	54	46
Adjusted segment EBITDA									8,233	8,218	7,915
Depreciation and amortization									2,131	1,966	1,904
Interest expense									1,690	1,707	1,665
Losses (gains) on sales of facilities	1	4	$ 1	1	15	2	$ 4	2	(8)	(23)	5
Losses on retirement of debt		$ 25				$ 2			39	4	135
Legal claim costs (benefits)										(246)	249
Income before income taxes									4,381	4,810	3,957
Assets	36,593				33,758				36,593	33,758	32,744
Goodwill and other intangible assets, Beginning Balance				6,704				6,731	6,704	6,731	6,416
Goodwill and other intangible assets, Acquisitions									693	41	345
Goodwill and other intangible assets, Foreign currency translation and other										(68)	(30)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(3)
Goodwill and other intangible assets, Ending Balance	7,394				6,704				7,394	6,704	6,731
National Group [Member]
Segment Reporting Information [Line Items]
Revenues									20,772	19,845	18,756
Equity in earnings of affiliates									(21)	(20)	(7)
Adjusted segment EBITDA									4,600	4,565	4,271
Depreciation and amortization									867	806	769
Assets	13,097				12,320				13,097	12,320	11,332
Goodwill and other intangible assets, Beginning Balance				1,458				1,481	1,458	1,481	1,170
Goodwill and other intangible assets, Acquisitions									19		318
Goodwill and other intangible assets, Foreign currency translation and other										(23)	(7)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(3)
Goodwill and other intangible assets, Ending Balance	1,474				1,458				1,474	1,458	1,481
American Group [Member]
Segment Reporting Information [Line Items]
Revenues									20,912	19,670	18,875
Equity in earnings of affiliates									(37)	(38)	(32)
Adjusted segment EBITDA									4,231	4,173	4,207
Depreciation and amortization									986	908	885
Assets	18,136				16,208				18,136	16,208	15,240
Goodwill and other intangible assets, Beginning Balance				4,661				4,638	4,661	4,638	4,614
Goodwill and other intangible assets, Acquisitions									612	33	27
Goodwill and other intangible assets, Foreign currency translation and other										(10)	(3)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									(8)
Goodwill and other intangible assets, Ending Balance	5,265				4,661				5,265	4,661	4,638
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues									1,930	1,975	2,047
Equity in earnings of affiliates									13	4	(7)
Adjusted segment EBITDA									(598)	(520)	(563)
Depreciation and amortization									278	252	250
Assets	5,360				5,230				5,360	5,230	6,172
Goodwill and other intangible assets, Beginning Balance				$ 585				$ 612	585	612	632
Goodwill and other intangible assets, Acquisitions									62	8
Goodwill and other intangible assets, Foreign currency translation and other										(35)	(20)
Goodwill and other intangible assets, Sales, foreign currency translation, amortization and other									8
Goodwill and other intangible assets, Ending Balance	$ 655				$ 585				$ 655	$ 585	$ 612