Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	$ 2,164	$ 3,176
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	34	39
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	709	1,521
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	24	42
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,873	4,697
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	58	81
Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	2,105	3,109
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	27	28
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	706	1,510
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	21	34
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,811	4,619
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	48	62
Equity Securities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	59	67
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	7	11
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	3	11
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	3	8
Investment securities, Continuous unrealized loss position, Fair Value, Total	62	78
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	10	19
Tax Exempt [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	1,103	422
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	9	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	219	305
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	15	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,322	727
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	24	16
Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	1,002	2,687
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	18	25
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	487	1,205
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	6	21
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,489	3,892
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	24	46
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	170	936
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	10	36
Investment securities, Continuous unrealized loss position, Fair Value, Total	180	972
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	1	1
Taxable [Member] | Corporate Bonds [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	474	1,327
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	15	23
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	311	888
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	4	17
Investment securities, Continuous unrealized loss position, Fair Value, Total	785	2,215
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	19	40
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	344	318
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	77	207
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation		2
Investment securities, Continuous unrealized loss position, Fair Value, Total	421	525
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	2	3
Residential Mortgage-backed Securities [Member] | Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	6	7
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	6	7
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	14	106
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	83	67
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	97	173
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 1