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	Sep. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,237	$ 223
Less than 12 months Unrealized Losses	64	12
12 months or more Fair Value	43	17
12 months or more Unrealized Losses	3	1
Total Fair Value	1,280	240
Total Unrealized Losses	67	13
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,159	112
Less than 12 months Unrealized Losses	61	2
12 months or more Fair Value	43	17
12 months or more Unrealized Losses	3	1
Total Fair Value	1,202	129
Total Unrealized Losses	64	3
Fixed Maturities | States, Municipalities And Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	371	53
Less than 12 months Unrealized Losses	17	1
12 months or more Fair Value	8	0
12 months or more Unrealized Losses	0	0
Total Fair Value	379	53
Total Unrealized Losses	17	1
Fixed Maturities | Foreign Government Debt Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	9
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	9
Total Unrealized Losses	0
Fixed Maturities | United States Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	1
Total Unrealized Losses	0
Fixed Maturities | Government-Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	190	1
Less than 12 months Unrealized Losses	19	0
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	190	1
Total Unrealized Losses	19	0
Fixed Maturities | Commercial Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	62
Less than 12 months Unrealized Losses	3
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	62
Total Unrealized Losses	3
Fixed Maturities | Corporate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	526	58
Less than 12 months Unrealized Losses	22	1
12 months or more Fair Value	35	17
12 months or more Unrealized Losses	3	1
Total Fair Value	561	75
Total Unrealized Losses	25	2
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	78	111
Less than 12 months Unrealized Losses	3	10
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	78	111
Total Unrealized Losses	3	10
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	74	107
Less than 12 months Unrealized Losses	3	9
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	74	107
Total Unrealized Losses	3	9
Equity Securities | Preferred Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	4	4
Less than 12 months Unrealized Losses	0	1
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	4	4
Total Unrealized Losses	$ 0	$ 1