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, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 354	$ 729
Less than 12 months Unrealized Losses	5	48
12 months or more Fair Value	53	88
12 months or more Unrealized Losses	3	5
Total Fair Value	407	817
Total Unrealized Losses	8	53
Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	239	391
Less than 12 months Unrealized Losses	3	13
12 months or more Fair Value	49	69
12 months or more Unrealized Losses	3	5
Total Fair Value	288	460
Total Unrealized Losses	6	18
Debt Securities | US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	53
Less than 12 months Unrealized Losses	1
12 months or more Fair Value	4	12
Total Fair Value	57	12
Total Unrealized Losses	1
Debt Securities | US Government Agencies Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	2	1
Total Fair Value	2	1
Debt Securities | US Government-sponsored Enterprises Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	31	10
Total Fair Value	31	10
Debt Securities | Corporate Bond Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	153	380
Less than 12 months Unrealized Losses	2	13
12 months or more Fair Value	45	57
12 months or more Unrealized Losses	3	5
Total Fair Value	198	437
Total Unrealized Losses	5	18
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	115	338
Less than 12 months Unrealized Losses	2	35
12 months or more Fair Value	4	19
Total Fair Value	119	357
Total Unrealized Losses	2	35
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	110	333
Less than 12 months Unrealized Losses	2	35
Total Fair Value	110	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