Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 715	$ 633	$ 930
Less than 12 months Unrealized Losses	(11)	(5)	(7)
12 months or more Fair Value		47	40
12 months or more Unrealized Losses		(5)	(3)
Total Fair Value	715	680	970
Total Unrealized Losses	(11)	(10)	(10)
US Treasury and Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		70
12 months or more Fair Value		1
Total Fair Value		71
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	405	34	606
Less than 12 months Unrealized Losses	(5)	(1)	(4)
12 months or more Fair Value		6
Total Fair Value	405	40	606
Total Unrealized Losses	(5)	(1)	(4)
Other Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	310	523	324
Less than 12 months Unrealized Losses	(6)	(4)	(3)
12 months or more Fair Value		38	37
12 months or more Unrealized Losses		(5)	(3)
Total Fair Value	310	561	361
Total Unrealized Losses	(6)	(9)	(6)
Equity Securities, Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		6
Total Fair Value		$ 6