Invested Assets and Related Income - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	$ 1,378	$ 1,430
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	31	65
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	236	676
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	29	79
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,614	2,106
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	60	144
Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	1,196	1,199
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	10	20
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	173	477
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	16	53
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,369	1,676
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	26	73
Equity securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	182	231
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	21	45
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	63	199
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	13	26
Investment securities, Continuous unrealized loss position, Fair Value, Total	245	430
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	34	71
Tax Exempt [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	344	81
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	6	1
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	104	240
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	13	36
Investment securities, Continuous unrealized loss position, Fair Value, Total	448	321
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	19	37
Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	852	1,118
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	4	19
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	69	237
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	3	17
Investment securities, Continuous unrealized loss position, Fair Value, Total	921	1,355
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	7	36
U.S. government and government agency and authority obligations [Member] | Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	28	19
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	20	18
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, Fair Value, Total	48	37
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total		2
Corporate bonds [Member] | Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	289	489
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	2	14
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	14	176
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	1	10
Investment securities, Continuous unrealized loss position, Fair Value, Total	303	665
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	3	24
Foreign government and government agency obligations [Member] | Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	429	499
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	13	21
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, Fair Value, Total	442	520
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	1	2
Residential mortgage-backed securities [Member] | Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	1	77
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation		2
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	19	22
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	2	5
Investment securities, Continuous unrealized loss position, Fair Value, Total	20	99
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	2	7
Commercial mortgage-backed securities [Member] | Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	105	34
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	3
Investment securities, Continuous unrealized loss position, Fair Value, Total	108	34
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 1