Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	$ 3,649	$ 3,176
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	78	39
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	870	1,521
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	35	42
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,519	4,697
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	113	81
Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	3,511	3,109
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	51	28
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	862	1,510
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	31	34
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,373	4,619
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	82	62
Equity Securities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	138	67
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	27	11
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	8	11
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	4	8
Investment securities, Continuous unrealized loss position, Fair Value, Total	146	78
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	31	19
Tax Exempt [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	1,287	422
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	16	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	249	305
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	13	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	1,536	727
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	29	16
Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	2,224	2,687
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	35	25
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	613	1,205
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	18	21
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,837	3,892
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	53	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	52	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	62	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	1,695	1,327
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	30	23
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	441	888
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	15	17
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,136	2,215
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	45	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	430	318
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	125	207
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	2	2
Investment securities, Continuous unrealized loss position, Fair Value, Total	555	525
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	5	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	5	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	5	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	47	106
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	32	67
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, Fair Value, Total	79	173
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 1