Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	$ 7,340	$ 1,378
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	262	31
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	214	236
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	21	29
Investment securities, Continuous unrealized loss position, Fair Value, Total	7,554	1,614
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	283	60
Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	7,272	1,196
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	256	10
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	204	173
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	17	16
Investment securities, Continuous unrealized loss position, Fair Value, Total	7,476	1,369
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	273	26
Equity securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	68	182
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	6	21
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	10	63
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	4	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	78	245
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	10	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	2,855	344
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	146	6
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	126	104
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	13	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,981	448
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	159	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	4,417	852
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	110	4
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	78	69
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	4	3
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,495	921
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	114	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	146	28
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	4
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	19	20
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	165	48
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	5
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	2,616	289
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	83	2
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	13	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	2,629	303
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	84	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	1,440	429
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	20	1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	9	13
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,449	442
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	21	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	7	1
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	16	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	23	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	208	105
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	3	1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	21	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation
Investment securities, Continuous unrealized loss position, Fair Value, Total	229	108
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 3	$ 1