Invested Assets - Amortized Cost and Fair Value of Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-For-Sale Securities [Line Items]
Tax exempt, Amortized Cost	$ 18,353	$ 18,410
Taxable, Amortized Cost	17,484	16,988
Tax exempt, Gross Unrealized Appreciation	953	1,522
Taxable, Gross Unrealized Appreciation	783	1,182
Tax exempt, Gross Unrealized Depreciation	159	19
Taxable, Gross Unrealized Depreciation	114	7
Tax exempt, Fair Value	19,147	19,913
Taxable, Fair Value	18,153	18,163
Total fixed maturities, Amortized Cost	35,837	35,398
Total fixed maturities, Gross Unrealized Appreciation	1,736	2,704
Total fixed maturities, Gross Unrealized Depreciation	273	26
Total fixed maturities, Fair Value	37,300	38,076
Equity securities, Amortized Cost	1,281	1,244
Equity securities, Gross Unrealized Appreciation	574	453
Equity securities, Gross Unrealized Depreciation	10	34
Equity securities, Fair Value	1,845	1,663
U.S. government and government agency and authority obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	779	973
Taxable, Gross Unrealized Appreciation	40	66
Taxable, Gross Unrealized Depreciation	5
Taxable, Fair Value	814	1,039
Corporate bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	8,126	7,331
Taxable, Gross Unrealized Appreciation	401	609
Taxable, Gross Unrealized Depreciation	84	3
Taxable, Fair Value	8,443	7,937
Foreign government and government agency obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	6,813	6,614
Taxable, Gross Unrealized Appreciation	270	395
Taxable, Gross Unrealized Depreciation	21	1
Taxable, Fair Value	7,062	7,008
Residential mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	336	421
Taxable, Gross Unrealized Appreciation	26	36
Taxable, Gross Unrealized Depreciation	1	2
Taxable, Fair Value	361	455
Commercial mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	1,430	1,649
Taxable, Gross Unrealized Appreciation	46	76
Taxable, Gross Unrealized Depreciation	3	1
Taxable, Fair Value	$ 1,473	$ 1,724