Loans - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans acquired in FDIC-assisted transactions	$ 460,700,000	$ 653,400,000	$ 507,700,000
Nonaccrual loans	200,000
Loan-to-value ratio	110.00%
Modified loans	27,400,000		40,300,000
Troubled debt restructurings	23,300,000	31,700,000	23,900,000
Troubled debt restructurings, previous charge-offs	2,600,000	2,300,000	1,900,000
Allowance for loan losses allocated to troubled debt restructurings	591,000	3,200,000	640,000
Troubled debt restructurings, provision for loan loss expense	320,000	282,000	2,600,000
Troubled Debt Restructuring [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 100,000