Invested Assets - Amortized Cost and Fair Value of Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-For-Sale Securities [Line Items]
Tax exempt, Amortized Cost	$ 18,333	$ 18,410
Taxable, Amortized Cost	17,401	16,988
Tax exempt, Gross Unrealized Appreciation	1,406	1,522
Taxable, Gross Unrealized Appreciation	1,116	1,182
Tax exempt, Gross Unrealized Depreciation	30	19
Taxable, Gross Unrealized Depreciation	10	7
Tax exempt, Fair Value	19,709	19,913
Taxable, Fair Value	18,507	18,163
Total fixed maturities, Amortized Cost	35,734	35,398
Total fixed maturities, Gross Unrealized Appreciation	2,522	2,704
Total fixed maturities, Gross Unrealized Depreciation	40	26
Total fixed maturities, Fair Value	38,216	38,076
Equity securities, Amortized Cost	1,227	1,244
Equity securities, Gross Unrealized Appreciation	635	453
Equity securities, Gross Unrealized Depreciation	14	34
Equity securities, Fair Value	1,848	1,663
U.S. government and government agency and authority obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	961	973
Taxable, Gross Unrealized Appreciation	65	66
Taxable, Fair Value	1,026	1,039
Corporate bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	7,949	7,331
Taxable, Gross Unrealized Appreciation	579	609
Taxable, Gross Unrealized Depreciation	6	3
Taxable, Fair Value	8,522	7,937
Foreign government and government agency obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	6,592	6,614
Taxable, Gross Unrealized Appreciation	369	395
Taxable, Gross Unrealized Depreciation	2	1
Taxable, Fair Value	6,959	7,008
Residential mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	374	421
Taxable, Gross Unrealized Appreciation	33	36
Taxable, Gross Unrealized Depreciation	1	2
Taxable, Fair Value	406	455
Commercial mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	1,525	1,649
Taxable, Gross Unrealized Appreciation	70	76
Taxable, Gross Unrealized Depreciation	1	1
Taxable, Fair Value	$ 1,594	$ 1,724