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, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 419	$ 223
Less than 12 months Unrealized Losses	5	12
12 months or more Fair Value	13	17
12 months or more Unrealized Losses	1	1
Total Fair Value	432	240
Total Unrealized Losses	6	13
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	344	112
Less than 12 months Unrealized Losses	3	2
12 months or more Fair Value	13	17
12 months or more Unrealized Losses	1	1
Total Fair Value	357	129
Total Unrealized Losses	4	3
Fixed Maturities | States, Municipalities And Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	103	53
Less than 12 months Unrealized Losses	1	1
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	103	53
Total Unrealized Losses	1	1
Fixed Maturities | Government-Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	107	1
Less than 12 months Unrealized Losses	1	0
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	107	1
Total Unrealized Losses	1	0
Fixed Maturities | Commercial Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	16
Less than 12 months Unrealized Losses	0
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	16
Total Unrealized Losses	0
Fixed Maturities | Corporate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	118	58
Less than 12 months Unrealized Losses	1	1
12 months or more Fair Value	13	17
12 months or more Unrealized Losses	1	1
Total Fair Value	131	75
Total Unrealized Losses	2	2
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	75	111
Less than 12 months Unrealized Losses	2	10
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	75	111
Total Unrealized Losses	2	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	2	9
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	74	107
Total Unrealized Losses	2	9
Equity Securities | Preferred Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1	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	1	4
Total Unrealized Losses	$ 0	$ 1