Loans and Allowances for Loan Losses - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest that would have been recognized on nonaccrual loans	$ 1,900,000
Interest recovered on nonaccrual loans	600,000
Non-Accrual Loans	136,350,000	137,615,000
Troubled Debt Restructuring	34,400,000	32,200,000
Minimum Bank Portfolio loan need to be reviewed	1,000,000
Accruing Loans Period	90 days
Restructured Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accrual Loans	$ 19,800,000