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, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 326	$ 729
Less than 12 months Unrealized Losses	5	48
12 months or more Fair Value	67	88
12 months or more Unrealized Losses	4	5
Total Fair Value	393	817
Total Unrealized Losses	9	53
Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	254	391
Less than 12 months Unrealized Losses	3	13
12 months or more Fair Value	60	69
12 months or more Unrealized Losses	4	5
Total Fair Value	314	460
Total Unrealized Losses	7	18
Debt Securities | US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	35
12 months or more Fair Value	1	12
Total Fair Value	36	12
Debt Securities | US Government Agencies Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1	1
Total Fair Value	1	1
Debt Securities | US Government-sponsored Enterprises Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	25	10
Total Fair Value	25	10
Debt Securities | Corporate Bond Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	193	380
Less than 12 months Unrealized Losses	3	13
12 months or more Fair Value	59	57
12 months or more Unrealized Losses	4	5
Total Fair Value	252	437
Total Unrealized Losses	7	18
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	72	338
Less than 12 months Unrealized Losses	2	35
12 months or more Fair Value	7	19
Total Fair Value	79	357
Total Unrealized Losses	2	35
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	67	333
Less than 12 months Unrealized Losses	2	35
12 months or more Fair Value	3
Total Fair Value	70	333
Total Unrealized Losses	2	35
Equity Securities | Preferred Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	5	5
12 months or more Fair Value	4	19
Total Fair Value	$ 9	$ 24