Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 749	$ 2,129	$ 1,072
Less than 12 months Unrealized Losses	(6)	(42)	(4)
12 months or more Fair Value	59	52	54
12 months or more Unrealized Losses	(6)	(5)	(5)
Total Fair Value	808	2,181	1,126
Total Unrealized Losses	(12)	(47)	(9)
US Treasury and Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	100		75
12 months or more Fair Value		1	1
Total Fair Value	100	1	76
US States and Political Subdivisions Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value		11	1
12 months or more Fair Value	3	4	3
Total Fair Value	3	15	4
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	27	1,555	995
Less than 12 months Unrealized Losses		(32)	(4)
12 months or more Fair Value	11
12 months or more Unrealized Losses	(1)
Total Fair Value	38	1,555	995
Total Unrealized Losses	(1)	(32)	(4)
Other Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	622	563	1
Less than 12 months Unrealized Losses	(6)	(10)
12 months or more Fair Value	45	47	50
12 months or more Unrealized Losses	(5)	(5)	(5)
Total Fair Value	667	610	51
Total Unrealized Losses	$ (11)	$ (15)	$ (5)