FDIC loss share (expense) income - Additional Information (Detail) (Federal Deposit Insurance Corporation [Member])	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Federal Deposit Insurance Corporation [Member]
F D I C Loss Share Expense Income [Line Items]
Loss Sharing Agreement Allocation Percentage	80	80	80
Limited Period For The F D I C Loss Sharing Agreements Covered For Interest Not Collected	approximately 90 days