Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 149	$ 729
Less than 12 months Unrealized Losses	4	48
12 months or more Fair Value	31	88
12 months or more Unrealized Losses	2	5
Total Fair Value	180	817
Total Unrealized Losses	6	53
Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	102	391
Less than 12 months Unrealized Losses	2	13
12 months or more Fair Value	31	69
12 months or more Unrealized Losses	2	5
Total Fair Value	133	460
Total Unrealized Losses	4	18
Debt Securities | US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	18
12 months or more Fair Value		12
Total Fair Value	18	12
Debt Securities | US Government-sponsored Enterprises Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	10	10
Total Fair Value	10	10
Debt Securities | Corporate Bond Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	74	380
Less than 12 months Unrealized Losses	2	13
12 months or more Fair Value	31	57
12 months or more Unrealized Losses	2	5
Total Fair Value	105	437
Total Unrealized Losses	4	18
Debt Securities | US Government Agencies Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		1
Total Fair Value		1
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	47	338
Less than 12 months Unrealized Losses	2	35
12 months or more Fair Value		19
Total Fair Value	47	357
Total Unrealized Losses	2	35
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	41	333
Less than 12 months Unrealized Losses	2	35
Total Fair Value	41	333
Total Unrealized Losses	2	35
Equity Securities | Preferred Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	6	5
12 months or more Fair Value		19
Total Fair Value	$ 6	$ 24