Segment and Related Information - Summary information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 21,829	$ 22,650	$ 21,829	$ 20,859
Goodwill, Beginning Balance		6,247	6,215
Acquired goodwill		248	39
Divested goodwill		(25)	(1)
Impairments	(34)	(6)	(34)	(12)
Foreign currency translation		(34)	28
Goodwill, Ending Balance	6,247	6,430	6,247	6,215
Tier 1 [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		2,221	2,203
Acquired goodwill		88	12
Divested goodwill		(6)
Foreign currency translation		(7)	6
Goodwill, Ending Balance	2,221	2,296	2,221	2,203
Tier 2 [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		1,237	1,196
Acquired goodwill		17	20
Divested goodwill			(1)
Foreign currency translation		(27)	22
Goodwill, Ending Balance	1,237	1,227	1,237	1,196
Tier 3 [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		2,668	2,661
Acquired goodwill		142	7
Goodwill, Ending Balance	2,668	2,810	2,668	2,661
Other [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		121	155
Acquired goodwill		1
Divested goodwill		(19)
Impairments		(6)	(34)
Goodwill, Ending Balance	121	97	121	155
Operating Segments [Member]
Segment Reporting Information [Line Items]
Assets	22,116	22,896	22,116	21,137
Operating Segments [Member] \| Tier 1 [Member]
Segment Reporting Information [Line Items]
Assets	6,528	6,958	6,528	6,188
Operating Segments [Member] \| Tier 2 [Member]
Segment Reporting Information [Line Items]
Assets	3,749	3,761	3,749	3,562
Operating Segments [Member] \| Tier 3 [Member]
Segment Reporting Information [Line Items]
Assets	8,727	9,119	8,727	8,497
Operating Segments [Member] \| Other [Member]
Segment Reporting Information [Line Items]
Assets	1,785	1,571	1,785	1,489
Elimination of Intercompany Investments and Advances [Member]
Segment Reporting Information [Line Items]
Assets	$ (287)	$ (246)	$ (287)	$ (278)

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
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