Invested Assets and Related Income - Amortized Cost and Fair Value of Fixed Maturities and Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Tax exempt, Amortized Cost	$ 18,410	$ 18,786
Taxable, Amortized Cost	16,988	15,976
Tax exempt, Gross Unrealized Appreciation	1,522	1,462
Taxable, Gross Unrealized Appreciation	1,182	1,033
Tax exempt, Gross Unrealized Depreciation	19	37
Taxable, Gross Unrealized Depreciation	7	36
Tax exempt, Fair Value	19,913	20,211
Taxable, Fair Value	18,163	16,973
Total fixed maturities, Amortized Cost	35,398	34,762
Total fixed maturities, Gross Unrealized Appreciation	2,704	2,495
Total fixed maturities, Gross Unrealized Depreciation	26	73
Total fixed maturities, Fair Value	38,076	37,184
Equity Securities, Amortized Cost	1,244	1,264
Equity securities, Gross unrealized appreciation	453	319
Equity securities, Gross Unrealized Depreciation	34	71
Equity securities, Fair Value	1,663	1,512
U.S. government and government agency and authority obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	973	813
Taxable, Gross Unrealized Appreciation	66	57
Taxable, Gross Unrealized Depreciation		2
Taxable, Fair Value	1,039	868
Corporate bonds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	7,331	6,049
Taxable, Gross Unrealized Appreciation	609	440
Taxable, Gross Unrealized Depreciation	3	24
Taxable, Fair Value	7,937	6,465
Foreign government and government agency obligations [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	6,614	6,409
Taxable, Gross Unrealized Appreciation	395	416
Taxable, Gross Unrealized Depreciation	1	2
Taxable, Fair Value	7,008	6,823
Residential mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	421	821
Taxable, Gross Unrealized Appreciation	36	41
Taxable, Gross Unrealized Depreciation	2	7
Taxable, Fair Value	455	855
Commercial mortgage-backed securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Taxable, Amortized Cost	1,649	1,884
Taxable, Gross Unrealized Appreciation	76	79
Taxable, Gross Unrealized Depreciation	1	1
Taxable, Fair Value	$ 1,724	$ 1,962