Schedule I - Parent Company Financial Information (Condensed Statements of Income) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Total revenues	$ 12,328	$ 12,238	$ 12,222	$ 11,712	$ 10,187	$ 10,319	$ 10,321	$ 10,486	$ 48,500	$ 41,313	$ 39,126
Expenses:
Operating costs									7,639	6,355	5,830
Depreciation									328	309	263
Interest									192	140	105
Benefit for income taxes									1,023	690	689
Net income	145	290	344	368	(30)	368	420	473	1,147	1,231	1,222
Parent Company
Revenues:
Management fees charged to operating subsidiaries									1,509	1,370	1,220
Investment and other income, net									4	3	5
Total revenues									1,513	1,373	1,225
Expenses:
Operating costs									1,434	1,366	1,229
Depreciation									212	193	172
Interest									192	141	106
Total operating expense									1,838	1,700	1,507
Loss before income taxes and equity in net earnings of subsidiaries									(325)	(327)	(282)
Benefit for income taxes									(81)	(107)	(102)
Loss before equity in net earnings of subsidiaries									(244)	(220)	(180)
Equity in net earnings of subsidiaries									1,391	1,451	1,402
Net income									$ 1,147	$ 1,231	$ 1,222