Invested Assets - Fair Value and Gross Unrealized Depreciation of Investments in Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	$ 6,561	$ 3,176
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	101	39
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	799	1,521
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	37	42
Investment securities, Continuous unrealized loss position, Fair Value, Total	7,360	4,697
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	138	81
Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	6,462	3,109
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	90	28
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	796	1,510
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	35	34
Investment securities, Continuous unrealized loss position, Fair Value, Total	7,258	4,619
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	125	62
Equity Securities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	99	67
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	11	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	2	8
Investment securities, Continuous unrealized loss position, Fair Value, Total	102	78
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	13	19
Tax Exempt [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	3,868	422
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	59	3
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	251	305
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	20	13
Investment securities, Continuous unrealized loss position, Fair Value, Total	4,119	727
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	79	16
Taxable [Member] | Fixed Maturities [Member]
Investment Holdings [Line Items]
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Fair Value	2,594	2,687
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	31	25
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	545	1,205
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	15	21
Investment securities, Continuous unrealized loss position, Fair Value, Total	3,139	3,892
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	46	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	306	936
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	10	36
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1
Investment securities, Continuous unrealized loss position, Fair Value, Total	316	972
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	3	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	1,682	1,327
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	22	23
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	432	888
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	12	17
Investment securities, Continuous unrealized loss position, Fair Value, Total	2,114	2,215
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	34	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	510	318
Investment securities, Continuous unrealized loss position, Less Than 12 Months, Gross Unrealized Depreciation	6	1
Investment securities, Continuous unrealized loss position, 12 Months or More, Fair Value	61	207
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation	1	2
Investment securities, Continuous unrealized loss position, Fair Value, Total	571	525
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	7	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	96	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	37	67
Investment securities, Continuous unrealized loss position, 12 Months or More, Gross Unrealized Depreciation		1
Investment securities, Continuous unrealized loss position, Fair Value, Total	133	173
Investment securities, Continuous unrealized loss position, Gross Unrealized Depreciation, Total	$ 1	$ 1