Fair Value and Gross Unrealized Losses on Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 513	$ 633	$ 2,378
Less than 12 months Unrealized Losses	(6)	(5)	(51)
12 months or more Fair Value	13	47	42
12 months or more Unrealized Losses		(5)	(3)
Total Fair Value	526	680	2,420
Total Unrealized Losses		(10)	(54)
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	85		5
12 months or more Fair Value	1	2	3
Total Fair Value	86	2	8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	78	34	1,807
Less than 12 months Unrealized Losses	(1)	(1)	(39)
12 months or more Fair Value	2	6
Total Fair Value	80	40	1,807
Total Unrealized Losses	(1)	(1)	(39)
Other Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	337	523	511
Less than 12 months Unrealized Losses	(5)	(4)	(11)
12 months or more Fair Value	10	38	38
12 months or more Unrealized Losses		(5)	(3)
Total Fair Value	347	561	549
Total Unrealized Losses	(5)	(9)	(14)
Equity Securities, Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	13	6	5
Less than 12 months Unrealized Losses			(1)
Total Fair Value	13	6	5
Total Unrealized Losses			(1)
US Government-sponsored Enterprises Debt Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value			50
Total Fair Value			$ 50