Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 633	$ 2,129
Less than 12 months Unrealized Losses	(5)	(42)
12 months or more Fair Value	47	52
12 months or more Unrealized Losses	(5)	(5)
Total Fair Value	680	2,181
Total Unrealized Losses	(10)	(47)
US Treasury and Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	70
12 months or more Fair Value	1	1
Total Fair Value	71	1
US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		11
12 months or more Fair Value	2	4
Total Fair Value	2	15
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	34	1,555
Less than 12 months Unrealized Losses	1	(32)
12 months or more Fair Value	6
12 months or more Unrealized Losses	0
Total Fair Value	40	1,555
Total Unrealized Losses	(1)	(32)
Other Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	523	563
Less than 12 months Unrealized Losses	(4)	(10)
12 months or more Fair Value	38	47
12 months or more Unrealized Losses	(5)	(5)
Total Fair Value	561	610
Total Unrealized Losses	(9)	(15)
Equity Securities, Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	6
Total Fair Value	$ 6