Securities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Available for sale securities reclassified to held to maturity securities	$ 1,500,000,000
Unrealized net holding gain on the available for sale securities on the date of transfer	39,000,000
Securities pledged to secure public deposits and securities sold under agreements to repurchase		$ 2,400,000,000	$ 3,000,000,000