Schedule I - Parent Company Financial Information (Condensed Statements of Income) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Total revenues	$ 12,878	$ 13,694	$ 14,007	$ 13,800	$ 13,361	$ 13,363	$ 13,732	$ 13,833	$ 54,379	$ 54,289	$ 48,500
Expenses:
Operating costs									7,277	7,318	7,639
Depreciation									388	354	328
Interest									189	186	192
Gain on sale of business									0	270	0
Benefit for income taxes									938	1,155	1,023
Net income	$ (401)	$ 450	$ 311	$ 254	$ 101	$ 314	$ 431	$ 430	614	1,276	1,147
Parent Company
Revenues:
Management fees charged to operating subsidiaries									1,683	1,469	1,509
Investment and other income, net									42	5	4
Total revenues									1,725	1,474	1,513
Expenses:
Operating costs									1,623	1,370	1,434
Depreciation									302	252	212
Interest									189	186	192
Total operating expense									2,114	1,808	1,838
Loss before gain on sale of business, income taxes and equity in net earnings of subsidiaries									(389)	(334)	(325)
Gain on sale of business									0	270	0
Loss before income taxes and equity in net earnings of subsidiaries									(389)	(64)	(325)
Benefit for income taxes									(107)	(70)	(81)
Loss before equity in net earnings of subsidiaries									(282)	6	(244)
Equity in net earnings of subsidiaries									896	1,270	1,391
Net income									$ 614	$ 1,276	$ 1,147