Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 519	$ 393
Less than 12 months Unrealized Losses	19	11
12 months or more Fair Value	295	446
12 months or more Unrealized Losses	5	11
Total Fair Value	814	839
Total Unrealized Losses	24	22
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	346	292
Less than 12 months Unrealized Losses	8	8
12 months or more Fair Value	280	429
12 months or more Unrealized Losses	5	10
Total Fair Value	626	721
Total Unrealized Losses	13	18
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	211	261
Less than 12 months Unrealized Losses	6	8
12 months or more Fair Value	64	90
12 months or more Unrealized Losses	2	3
Total Fair Value	275	351
Total Unrealized Losses	8	11
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	115	17
Less than 12 months Unrealized Losses	2	0
12 months or more Fair Value	58	135
12 months or more Unrealized Losses	0	2
Total Fair Value	173	152
Total Unrealized Losses	2	2
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	2	3
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	2	23
12 months or more Unrealized Losses	0	0
Total Fair Value	4	26
Total Unrealized Losses	0	0
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	18	11
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	156	181
12 months or more Unrealized Losses	3	5
Total Fair Value	174	192
Total Unrealized Losses	3	5
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	173	101
Less than 12 months Unrealized Losses	11	3
12 months or more Fair Value	15	17
12 months or more Unrealized Losses	0	1
Total Fair Value	188	118
Total Unrealized Losses	11	4
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	168	85
Less than 12 months Unrealized Losses	11	3
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	168	85
Total Unrealized Losses	11	3
Equity Securities | Nonredeemable preferred equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	5	16
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	15	17
12 months or more Unrealized Losses	0	1
Total Fair Value	20	33
Total Unrealized Losses	$ 0	$ 1