Schedule I - Parent Company Financial Information (Condensed Statements of Income) (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
Revenues:
Total revenues	$ 13,189	$ 13,282	$ 13,534	$ 13,762	$ 12,878	$ 13,694	$ 14,007	$ 13,800	$ 53,767	$ 54,379	$ 54,289
Expenses:
Operating costs									6,567	7,173	7,295
Merger termination fee and related costs, net									(936)	104	23
Depreciation									410	388	354
Interest									242	189	186
Total operating expenses									49,505	52,638	51,942
Gain on sale of business									0	0	270
Provision (benefit) for income taxes									1,572	938	1,155
Net income	$ 184	$ 499	$ 650	$ 1,115	$ (401)	$ 450	$ 311	$ 254	2,448	614	1,276
Parent Company
Revenues:
Management fees charged to operating subsidiaries									1,864	1,683	1,469
Investment and other income, net									57	42	5
Total revenues									1,921	1,725	1,474
Expenses:
Operating costs									1,801	1,519	1,347
Merger termination fee and related costs, net									(936)	104	23
Depreciation									332	302	252
Interest									243	189	186
Total operating expenses									1,440	2,114	1,808
Income (loss) before gain on sale of business, income taxes and equity in net earnings of subsidiaries									481	(389)	(334)
Gain on sale of business									0	0	270
Income (loss) before income taxes and equity in net earnings of subsidiaries									481	(389)	(64)
Provision (benefit) for income taxes									61	(107)	(70)
Income (loss) before equity in net earnings of subsidiaries									420	(282)	6
Equity in net earnings of subsidiaries									2,028	896	1,270
Net income									$ 2,448	$ 614	$ 1,276