Loans - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 RiskGrades	Mar. 31, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased non-covered loans	$ 643,100,000	$ 437,300,000	$ 674,200,000
Covered loans acquired in FDIC-assisted transactions	245,700,000	372,700,000	271,300,000
Number of risk grading system category	9
Modified loans	32,000,000	6,300,000
Troubled debt restructurings	1,519,000	2,241,000	1,212,000
Troubled debt restructurings, previous charge-offs	1,600,000	2,300,000	2,200,000
Allowance for loan losses allocated to troubled debt restructurings	1,600,000	422,000	231,000
Commitments to lend additional funds	0
Troubled debt restructurings principal balances	2,700,000	1,200,000
Allowance for loan losses adjustments	0	0
Troubled debt restructurings principal balances	1,500,000	2,200,000
Troubled debt restructurings, provision for loan loss expense	401,000	225,000	843,000
Purchased Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	17,308,000	15,318,000	18,249,000
Modified loans	5,433,000	10,126,000	10,347,000
Troubled debt restructurings	1,526,000	5,191,000	1,212,000
Troubled debt restructurings, previous charge-offs	0	0	29,000
Commitments to lend additional funds	0
Non-Covered Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings, provision for loan loss expense			84,000
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	38,359,000	59,781,000	35,412,000
Modified loans	5,277,000	24,249,000	8,534,000
Troubled debt restructurings	23,300,000	27,800,000	24,600,000
Troubled debt restructurings, previous charge-offs	1,100,000	3,200,000	1,800,000
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