Fair Values and Unrealized Losses by Investment Category and by the Duration of the Securities' Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 729	$ 1,154
Less than 12 months Unrealized Losses	48	44
12 months or more Fair Value	88	71
12 months or more Unrealized Losses	5	5
Total Fair Value	817	1,225
Total Unrealized Losses	53	49
Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	391	812
Less than 12 months Unrealized Losses	13	16
12 months or more Fair Value	69	48
12 months or more Unrealized Losses	5	4	25
Total Fair Value	460	860
Total Unrealized Losses	18	20
Debt Securities | US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		325
Less than 12 months Unrealized Losses		9
12 months or more Fair Value	12	9
12 months or more Unrealized Losses		1
Total Fair Value	12	334
Total Unrealized Losses		10
Debt Securities | US Government Agencies Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1
Total Fair Value	1
Debt Securities | US Government-sponsored Enterprises Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	10	133
Less than 12 months Unrealized Losses		1
Total Fair Value	10	133
Total Unrealized Losses		1
Debt Securities | Corporate Bond Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	380	354
Less than 12 months Unrealized Losses	13	6
12 months or more Fair Value	57	39
12 months or more Unrealized Losses	5	3
Total Fair Value	437	393
Total Unrealized Losses	18	9
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	338	342
Less than 12 months Unrealized Losses	35	28
12 months or more Fair Value	19	23
12 months or more Unrealized Losses		1	26
Total Fair Value	357	365
Total Unrealized Losses	35	29
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	333	337
Less than 12 months Unrealized Losses	35	28
Total Fair Value	333	337
Total Unrealized Losses	35	28
Equity Securities | Preferred Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	5	5
12 months or more Fair Value	19	23
12 months or more Unrealized Losses		1
Total Fair Value	24	28
Total Unrealized Losses		$ 1