Loans - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013 Category	Sep. 30, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans acquired in FDIC-assisted transactions	$ 417,600,000	$ 546,200,000	$ 507,700,000
Past due loan accrued	90 days
Nonaccrual loans	200,000
Number of risk grading system category	9
Loan-to-value ratio	110.00%
Modified loans	17,000,000	23,500,000
Troubled debt restructurings	20,200,000	23,300,000	23,900,000
Troubled debt restructurings, previous charge-offs	2,100,000	2,100,000	1,900,000
Allowance for loan losses allocated to troubled debt restructurings	412,000	676,000	640,000
Troubled debt restructurings, provision for loan loss expense	1,300,000	2,300,000	2,600,000
Troubled Debt Restructuring [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 100,000