Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 RiskGrades	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased non-covered loans	$ 674,200,000	$ 448,800,000
Covered loans acquired in FDIC-assisted transactions	271,300,000	390,200,000
Loans on nonaccrual status, excluding purchased non-covered and covered loans	21,700,000	29,200,000	38,900,000
Purchased non-covered loans on nonaccrual status	18,200,000	6,700,000
Troubled debt restructurings, provision for loan loss expense	843,000	1,500,000	2,600,000
Number of risk grading system category	9
Modified loans	29,100,000	30,400,000
Troubled debt restructurings	1,214,000	3,390,000
Troubled debt restructurings, previous charge-offs	2,200,000	2,100,000
Allowance for loan losses allocated to troubled debt restructurings	231,000	432,000
Commitments to lend additional funds	0
Troubled debt restructurings principal balances	3,100,000
Allowance for loan losses adjustments	232,000
Troubled debt restructurings principal balances		1,200,000
Deposits from principal officers, directors, and their affiliates	6,018,000	5,994,000
Non-Covered Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings, provision for loan loss expense	84,000	0	0
Purchased Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	18,249,000	6,659,000
Modified loans	10,347,000	12,903,000
Troubled debt restructurings	1,212,000	5,938,000
Troubled debt restructurings, previous charge-offs	29,000
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	35,412,000	69,152,000
Modified loans	8,534,000	20,990,000
Troubled debt restructurings	24,600,000	27,300,000
Troubled debt restructurings, previous charge-offs	1,800,000	1,600,000
Commitments to lend additional funds	0
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due loan accrued	90 days
Nonaccrual loans	100,000
Loan-to-value ratio	110.00%
Minimum [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 200,000