Loans - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Category	Jun. 30, 2012	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Covered loans acquired in FDIC-assisted transactions			$ 443,500,000	$ 601,700,000	$ 507,700,000
Past due loan accrued			90 days
Nonaccrual loans	200,000		200,000
Number of risk grading system category			9
Loan-to-value ratio	110.00%		110.00%
Modified loans	20,700,000		20,700,000		40,300,000
Troubled debt restructurings	20,600,000	28,200,000	20,600,000	28,200,000	23,900,000
Troubled debt restructurings, previous charge-offs			2,000,000	2,000,000	1,900,000
Allowance for loan losses allocated to troubled debt restructurings			482,000	868,000	640,000
Troubled debt restructurings, provision for loan loss expense	4,165,000	7,225,000	7,088,000	20,107,000	2,600,000
Troubled Debt Restructuring [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 100,000		$ 100,000