Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	$ 2,216	$ 1,378
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	36	31
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	201	236
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	18	29
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,417	1,614
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	54	60
Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	2,125	1,196
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	27	10
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	167	173
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	13	16
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,292	1,369
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	40	26
Equity securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	91	182
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	9	21
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	34	63
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	5	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	125	245
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	14	34
Tax exempt [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	1,065	344
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	19	6
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	114	104
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	11	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,179	448
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	30	19
Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	1,060	852
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	8	4
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	53	69
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	2	3
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,113	921
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	10	7
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 12 Months, Fair Value	18	28
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	20	20
Investment securities, Continuous unrealized loss position, Fair Value, Total	38	48
Corporate bonds [Member] | Taxable [Member] | Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	625	289
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	5	2
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	14	14
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	639	303
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	6	3
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 12 Months, Fair Value	289	429
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	2	1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value		13
Investment securities, Continuous unrealized loss position, Fair Value, Total	289	442
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	2	1
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 12 Months, Fair Value		1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	19	19
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1	2
Investment securities, Continuous unrealized loss position, Fair Value, Total	19	20
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	1	2
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 12 Months, Fair Value	128	105
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value		3
Investment securities, Continuous unrealized loss position, Fair Value, Total	128	108
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 1