Invested Assets - Amortized Cost and Fair Value of Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-For-Sale Securities [Line Items]
Tax exempt, Amortized Cost	$ 18,210	$ 18,410
Taxable, Amortized Cost	18,036	16,988
Tax exempt, Gross Unrealized Appreciation	905	1,522
Taxable, Gross Unrealized Appreciation	734	1,182
Tax exempt, Gross Unrealized Depreciation	164	19
Taxable, Gross Unrealized Depreciation	111	7
Tax exempt, Fair Value	18,951	19,913
Taxable, Fair Value	18,659	18,163
Total fixed maturities, Amortized Cost	36,246	35,398
Total fixed maturities, Gross Unrealized Appreciation	1,639	2,704
Total fixed maturities, Gross Unrealized Depreciation	275	26
Total fixed maturities, Fair Value	37,610	38,076
Equity securities, Amortized Cost	1,163	1,244
Equity securities, Gross Unrealized Appreciation	665	453
Equity securities, Gross Unrealized Depreciation	7	34
Equity securities, Fair Value	1,821	1,663
U.S. government and government agency and authority obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	790	973
Taxable, Gross Unrealized Appreciation	35	66
Taxable, Gross Unrealized Depreciation	7
Taxable, Fair Value	818	1,039
Corporate bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	8,838	7,331
Taxable, Gross Unrealized Appreciation	399	609
Taxable, Gross Unrealized Depreciation	77	3
Taxable, Fair Value	9,160	7,937
Foreign government and government agency obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	6,773	6,614
Taxable, Gross Unrealized Appreciation	238	395
Taxable, Gross Unrealized Depreciation	24	1
Taxable, Fair Value	6,987	7,008
Residential mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	302	421
Taxable, Gross Unrealized Appreciation	23	36
Taxable, Gross Unrealized Depreciation	1	2
Taxable, Fair Value	324	455
Commercial mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	1,333	1,649
Taxable, Gross Unrealized Appreciation	39	76
Taxable, Gross Unrealized Depreciation	2	1
Taxable, Fair Value	$ 1,370	$ 1,724