Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,813	$ 393
Less than 12 months Unrealized Losses	41	11
12 months or more Fair Value	280	446
12 months or more Unrealized Losses	10	11
Total Fair Value	2,093	839
Total Unrealized Losses	51	22
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,582	292
Less than 12 months Unrealized Losses	32	8
12 months or more Fair Value	275	429
12 months or more Unrealized Losses	10	10
Total Fair Value	1,857	721
Total Unrealized Losses	42	18
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	715	261
Less than 12 months Unrealized Losses	14	8
12 months or more Fair Value	65	90
12 months or more Unrealized Losses	2	3
Total Fair Value	780	351
Total Unrealized Losses	16	11
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	693	17
Less than 12 months Unrealized Losses	14	0
12 months or more Fair Value	56	135
12 months or more Unrealized Losses	1	2
Total Fair Value	749	152
Total Unrealized Losses	15	2
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	74	3
Less than 12 months Unrealized Losses	2	0
12 months or more Fair Value	2	23
12 months or more Unrealized Losses	0	0
Total Fair Value	76	26
Total Unrealized Losses	2	0
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	90	11
Less than 12 months Unrealized Losses	2	0
12 months or more Fair Value	152	181
12 months or more Unrealized Losses	7	5
Total Fair Value	242	192
Total Unrealized Losses	9	5
Fixed Maturities | Foreign government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	10
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	10
Total Unrealized Losses	0
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	231	101
Less than 12 months Unrealized Losses	9	3
12 months or more Fair Value	5	17
12 months or more Unrealized Losses	0	1
Total Fair Value	236	118
Total Unrealized Losses	9	4
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	165	85
Less than 12 months Unrealized Losses	8	3
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	165	85
Total Unrealized Losses	8	3
Equity Securities | Nonredeemable preferred equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	66	16
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	5	17
12 months or more Unrealized Losses	0	1
Total Fair Value	71	33
Total Unrealized Losses	$ 1	$ 1