Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended		9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Restructured Commercial Loan [Member]	Sep. 30, 2012 FDIC [Member]	Sep. 30, 2011 FDIC [Member]	Sep. 30, 2012 Loans [Member]	Sep. 30, 2011 Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in allowance for losses related to impairment loans						$ 37,000,000	$ 33,400,000
Increase (decrease) in indemnification asset				34,400,000	31,800,000
Interest that would have been recognized on nonaccrual loans	5,800,000
Interest recovered on nonaccrual loans	1,400,000
Non-accrual loans	157,127,000	103,270,000	21,600,000
Troubled Debt restructuring	32,300,000	18,100,000
Commercial substandard understated		91,600,000
Accruing loans Period	90 days
Loans held for sale	51,097,000	72,378,000
Problem commercial loans held for sale	4,900,000
Mortgage loans held for loan	$ 46,200,000