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	Jun. 30, 2013	Dec. 31, 2012
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 1,410	$ 223
Less than 12 months Unrealized Losses	68	12
12 months or more Fair Value	13	17
12 months or more Unrealized Losses	1	1
Total Fair Value	1,423	240
Total Unrealized Losses	69	13
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1,204	112
Less than 12 months Unrealized Losses	56	2
12 months or more Fair Value	13	17
12 months or more Unrealized Losses	1	1
Total Fair Value	1,217	129
Total Unrealized Losses	57	3
Fixed Maturities | States, Municipalities And Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	377	53
Less than 12 months Unrealized Losses	18	1
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	377	53
Total Unrealized Losses	18	1
Fixed Maturities | Foreign Government Debt Securities [Member]
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
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	185	1
Less than 12 months Unrealized Losses	13	0
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	185	1
Total Unrealized Losses	13	0
Fixed Maturities | Commercial Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	51
Less than 12 months Unrealized Losses	3
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	51
Total Unrealized Losses	3
Fixed Maturities | Corporate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	580	58
Less than 12 months Unrealized Losses	22	1
12 months or more Fair Value	13	17
12 months or more Unrealized Losses	1	1
Total Fair Value	593	75
Total Unrealized Losses	23	2
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	206	111
Less than 12 months Unrealized Losses	12	10
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	206	111
Total Unrealized Losses	12	10
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	202	107
Less than 12 months Unrealized Losses	12	9
12 months or more Fair Value	0
12 months or more Unrealized Losses	0
Total Fair Value	202	107
Total Unrealized Losses	12	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