Securities - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010 Maximum [Member]
Investment Securities [Line Items]
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
Proceeds and pay downs of available for sale securities		48,000,000	343,000,000	1,000,000
Securities pledged to secure public deposits and securities sold under agreements to repurchase		$ 2,600,000,000	$ 3,000,000,000