Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 603	$ 633	$ 749
Less than 12 months Unrealized Losses	(14)	(5)	(6)
12 months or more Fair Value	5	47	59
12 months or more Unrealized Losses		(5)	(6)
Total Fair Value	608	680	808
Total Unrealized Losses	(14)	(10)	(12)
US Treasury and Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		70	100
12 months or more Fair Value		1
Total Fair Value		71	100
US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
12 months or more Fair Value		2	3
Total Fair Value		2	3
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	185	34	27
Less than 12 months Unrealized Losses	(9)	(1)
12 months or more Fair Value	5	6	11
12 months or more Unrealized Losses			(1)
Total Fair Value	190	40	38
Total Unrealized Losses	(9)	(1)	(1)
Other Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	418	523	622
Less than 12 months Unrealized Losses	(5)	(4)	(6)
12 months or more Fair Value		38	45
12 months or more Unrealized Losses		(5)	(5)
Total Fair Value	418	561	667
Total Unrealized Losses	(5)	(9)	(11)
Equity Securities, Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		6
Total Fair Value		$ 6