Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,651	$ 393
Less than 12 months Unrealized Losses	104	11
12 months or more Fair Value	277	446
12 months or more Unrealized Losses	10	11
Total Fair Value	1,928	839
Total Unrealized Losses	114	22
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,097	292
Less than 12 months Unrealized Losses	47	8
12 months or more Fair Value	277	429
12 months or more Unrealized Losses	10	10
Total Fair Value	1,374	721
Total Unrealized Losses	57	18
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	729	261
Less than 12 months Unrealized Losses	42	8
12 months or more Fair Value	85	90
12 months or more Unrealized Losses	7	3
Total Fair Value	814	351
Total Unrealized Losses	49	11
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	227	17
Less than 12 months Unrealized Losses	3	0
12 months or more Fair Value	51	135
12 months or more Unrealized Losses	0	2
Total Fair Value	278	152
Total Unrealized Losses	3	2
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	57	3
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	2	23
12 months or more Unrealized Losses	0	0
Total Fair Value	59	26
Total Unrealized Losses	1	0
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	74	11
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	139	181
12 months or more Unrealized Losses	3	5
Total Fair Value	213	192
Total Unrealized Losses	4	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	554	101
Less than 12 months Unrealized Losses	57	3
12 months or more Fair Value	0	17
12 months or more Unrealized Losses	0	1
Total Fair Value	554	118
Total Unrealized Losses	57	4
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	511	85
Less than 12 months Unrealized Losses	56	3
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	511	85
Total Unrealized Losses	56	3
Equity Securities | Nonredeemable preferred equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	43	16
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	0	17
12 months or more Unrealized Losses	0	1
Total Fair Value	43	33
Total Unrealized Losses	$ 1	$ 1