Loans - Additional Information (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($) RiskGrades	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased non-covered loans	$ 808,300,000	$ 702,100,000	$ 674,200,000
Purchased loan pools not acquired in bank acquisitions or FDIC-assisted transactions	268,984,000	0	0
Residential mortgages on properties principal balances	263,800,000
Purchase premium paid at acquisition	5,200,000
Covered loans acquired in FDIC-assisted transactions	$ 209,600,000	331,300,000	271,300,000
Number of risk grading system category | RiskGrades	9
Troubled debt restructurings	$ 14,000,000	21,100,000	15,300,000
Troubled debt restructurings, previous charge-offs	1,600,000	3,000,000	2,200,000
Allowance for loan losses allocated to troubled debt restructurings	210,000	398,000	231,000
Commitments to lend additional funds	0
Troubled debt restructurings, excluding purchased non-covered and covered loans principal balances	782,000	1,700,000
Allowance for loan losses adjustments	0	0
Troubled debt restructurings principal balances	2,200,000	130,000
Loans 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	54,800,000	8,400,000
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	28,871,000	46,138,000	35,412,000
Modified loans	21,480,000	42,315,000	33,383,000
Troubled debt restructurings	19,600,000	23,700,000	24,600,000
Troubled debt restructurings, previous charge-offs	42,000	1,500,000	1,800,000
Commitments to lend additional funds	0
Allowance for loan losses adjustments	0	0
Troubled debt restructurings principal balances	297,000	1,100,000
Troubled debt restructuring loans modifications	1,212,000	2,577,000
Covered Loans [Member] | Loans 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	3,918,000	9,456,000	8,534,000
Purchased Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	17,444,000	15,770,000	18,249,000
Modified loans	25,182,000	27,044,000	32,022,000
Troubled debt restructurings	6,792,000	0	1,212,000
Troubled debt restructurings, previous charge-offs	632,000		29,000
Commitments to lend additional funds	0
Allowance for loan losses adjustments	0
Troubled debt restructurings principal balances	65,000	0
Troubled debt restructuring loans modifications	1,000,000
Purchased Non-Covered Loans [Member] | Loans 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	6,214,000	$ 9,087,000	$ 10,347,000
Transferred From Covered Loan To Purchased Non Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring loans modifications	$ 4,800,000
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
Minimum [Member] | Independent Third Party Loan Review [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan relationship with risk ratings reviewed annually	250,000
Amount of loan relationship with risk ratings reviewed annually	250,000
Minimum [Member] | Internal Loan Review Department [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan relationship with risk ratings reviewed annually	$ 1,000,000