Segment and Related Information - Summary information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Assets	$ 21,829	$ 21,829	$ 20,859	$ 20,367
Goodwill, Beginning Balance		6,215	5,984
Acquired goodwill		39	280
Divested goodwill		(1)	(1)
Impairments	(34)	(34)	(12)
Foreign currency translation		28	12
Other adjustments			(48)
Goodwill, Ending Balance	6,247	6,247	6,215
Tier 1 [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		2,203	2,190
Acquired goodwill		12	11
Foreign currency translation		6	2
Goodwill, Ending Balance	2,221	2,221	2,203
Tier 2 [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		1,196	1,176
Acquired goodwill		20	12
Divested goodwill		(1)
Foreign currency translation		22	10
Other adjustments			(2)
Goodwill, Ending Balance	1,237	1,237	1,196
Tier 3 [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		2,661	2,408
Acquired goodwill		7	254
Divested goodwill			(1)
Goodwill, Ending Balance	2,668	2,668	2,661
Other [Member]
Segment Reporting Information [Line Items]
Goodwill, Beginning Balance		155	210
Acquired goodwill			3
Impairments		(34)	(12)
Other adjustments			(46)
Goodwill, Ending Balance	121	121	155
Elimination of Intercompany Investments and Advances [Member]
Segment Reporting Information [Line Items]
Assets	$ (287)	$ (287)	$ (278)	$ (539)