Invested Assets - Amortized Cost and Fair Value of Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Tax exempt, Amortized Cost	$ 18,548	$ 18,786
Tax exempt, Gross Unrealized Appreciation	1,628	1,462
Tax exempt, Gross Unrealized Depreciation	19	37
Tax exempt, Fair Value	20,157	20,211
Taxable, Amortized Cost	16,836	15,976
Taxable, Gross Unrealized Appreciation	1,231	1,033
Taxable, Gross Unrealized Depreciation	7	36
Taxable, Fair Value	18,060	16,973
Total fixed maturities, Amortized Cost	35,384	34,762
Total fixed maturities, Gross Unrealized Appreciation	2,859	2,495
Total fixed maturities, Gross Unrealized Depreciation	26	73
Total fixed maturities, Fair Value	38,217	37,184
Equity Securities, Amortized Cost	1,234	1,264
Equity securities, Gross unrealized appreciation	413	319
Equity securities, Gross unrealized depreciation	34	71
Equity securities, Fair Value	1,613	1,512
U.S. government and government agency and authority obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	985	813
Taxable, Gross Unrealized Appreciation	72	57
Taxable, Gross Unrealized Depreciation	1	2
Taxable, Fair Value	1,056	868
Corporate bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	7,153	6,049
Taxable, Gross Unrealized Appreciation	613	440
Taxable, Gross Unrealized Depreciation	2	24
Taxable, Fair Value	7,764	6,465
Foreign government and government agency obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	6,528	6,409
Taxable, Gross Unrealized Appreciation	422	416
Taxable, Gross Unrealized Depreciation	1	2
Taxable, Fair Value	6,949	6,823
Residential mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	491	821
Taxable, Gross Unrealized Appreciation	38	41
Taxable, Gross Unrealized Depreciation	2	7
Taxable, Fair Value	527	855
Commercial mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	1,679	1,884
Taxable, Gross Unrealized Appreciation	86	79
Taxable, Gross Unrealized Depreciation	1	1
Taxable, Fair Value	$ 1,764	$ 1,962