Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 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	$ 7,212	$ 1,378
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	251	31
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	282	236
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	31	29
Investment securities, Continuous unrealized loss position, Fair Value, Total	7,494	1,614
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	282	60
Fixed maturities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	7,167	1,196
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	247	10
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	271	173
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	28	16
Investment securities, Continuous unrealized loss position, Fair Value, Total	7,438	1,369
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	275	26
Equity securities [Member]
Schedule of Available-For-Sale Securities [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	45	182
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	4	21
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	11	63
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	3	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	56	245
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	7	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	2,900	344
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	145	6
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	128	104
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	19	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	3,028	448
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	164	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	4,267	852
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	102	4
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	143	69
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	9	3
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,410	921
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	111	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	139	28
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	34	20
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	4
Investment securities, Continuous unrealized loss position, Fair Value, Total	173	48
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	7
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	2,512	289
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	75	2
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	42	14
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	2	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,554	303
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	77	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	1,519	429
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	22	1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	36	13
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	2
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,555	442
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	24	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	3	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	15	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	18	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	94	105
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	16	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation
Investment securities, Continuous unrealized loss position, Fair Value, Total	110	108
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 2	$ 1