Fair Values and Unrealized Losses by Investment Category and by the Duration of the Securities' Continuous Unrealized Loss Position (Detail) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 658	$ 1,154
Less than 12 months Unrealized Losses	26	44
12 months or more Fair Value	55	71
12 months or more Unrealized Losses	4	5
Total Fair Value	713	1,225
Total Unrealized Losses	30	49
Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	456	812
Less than 12 months Unrealized Losses	5	16
12 months or more Fair Value	37	48
12 months or more Unrealized Losses	3	4
Total Fair Value	493	860
Total Unrealized Losses	8	20
Debt Securities | US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	123	325
Less than 12 months Unrealized Losses	2	9
12 months or more Fair Value	9	9
12 months or more Unrealized Losses	1	1
Total Fair Value	132	334
Total Unrealized Losses	3	10
Debt Securities | US Government-sponsored Enterprises Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	77	133
Less than 12 months Unrealized Losses	1	1
Total Fair Value	77	133
Total Unrealized Losses	1	1
Debt Securities | Corporate Bond Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	256	354
Less than 12 months Unrealized Losses	2	6
12 months or more Fair Value	28	39
12 months or more Unrealized Losses	2	3
Total Fair Value	284	393
Total Unrealized Losses	4	9
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	202	342
Less than 12 months Unrealized Losses	21	28
12 months or more Fair Value	18	23
12 months or more Unrealized Losses	1	1
Total Fair Value	220	365
Total Unrealized Losses	22	29
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	197	337
Less than 12 months Unrealized Losses	21	28
Total Fair Value	197	337
Total Unrealized Losses	21	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	18	23
12 months or more Unrealized Losses	1	1
Total Fair Value	23	28
Total Unrealized Losses	$ 1	$ 1