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, 2014	Dec. 31, 2013
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	$ 3,176	$ 8,089
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	39	262
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	1,521	692
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	42	66
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,697	8,781
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	81	328
Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	3,109	8,048
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	28	259
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	1,510	692
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	34	66
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,619	8,740
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	62	325
Equity Securities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	67	41
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	11	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	11
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	8
Investment securities, Continuous unrealized loss position, Fair Value, Total	78	41
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	19	3
Tax Exempt [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	422	3,417
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	3	144
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	305	307
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	13	45
Investment securities, Continuous unrealized loss position, Fair Value, Total	727	3,724
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	16	189
Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	2,687	4,631
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	25	115
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	1,205	385
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	21	21
Investment securities, Continuous unrealized loss position, Fair Value, Total	3,892	5,016
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	46	136
Taxable [Member] | U.S. Government and Government Agency and Authority Obligations [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	936	213
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	1	6
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	36	35
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation		3
Investment securities, Continuous unrealized loss position, Fair Value, Total	972	248
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	1	9
Taxable [Member] | Corporate Bonds [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	1,327	2,526
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	23	76
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	888	222
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	17	12
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,215	2,748
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	40	88
Taxable [Member] | Foreign Government and Government Agency Obligations [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	318	1,735
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	1	32
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	207	75
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	2	3
Investment securities, Continuous unrealized loss position, Fair Value, Total	525	1,810
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	3	35
Residential Mortgage-backed Securities [Member] | Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value		4
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	7	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	7	18
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	1	1
Commercial Mortgage-backed Securities [Member] | Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	106	153
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	67	39
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1	2
Investment securities, Continuous unrealized loss position, Fair Value, Total	173	192
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 3