Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less than twelve months, Fair Value	$ 726	$ 1,430
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	23	65
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	271	676
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	37	79
Investment securities, Continuous unrealized loss position, Fair Value, Total	997	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	546	1,199
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	4	20
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	218	477
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	22	53
Investment securities, Continuous unrealized loss position, Fair Value, Total	764	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	180	231
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	19	45
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	53	199
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	15	26
Investment securities, Continuous unrealized loss position, Fair Value, Total	233	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	70	81
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	98	240
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	18	36
Investment securities, Continuous unrealized loss position, Fair Value, Total	168	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	476	1,118
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	3	19
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	120	237
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	4	17
Investment securities, Continuous unrealized loss position, Fair Value, Total	596	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	37	18
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	65	37
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	1	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	170	489
Investment securities, Continuous unrealized loss position, Less than twelve months, Gross Unrealized Depreciation	1	14
Investment securities, Continuous unrealized loss position, Twelve months or longer, Fair Value	49	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	219	665
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	2	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	244	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	12	21
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, Fair Value, Total	256	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		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	21	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	21	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	34	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	1
Investment securities, Continuous unrealized loss position, Twelve months or longer, Gross Unrealized Depreciation
Investment securities, Continuous unrealized loss position, Fair Value, Total	35	34
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 1