Loans - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2014 RiskGrades	Sep. 30, 2013	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased non-covered loans	$ 673,700,000		$ 673,700,000	$ 0	$ 448,800,000
Covered loans acquired in FDIC-assisted transactions			313,600,000	417,600,000	390,200,000
Number of risk grading system category			9
Modified loans	8,000,000		8,000,000	17,000,000
Troubled debt restructurings	20,500,000		20,500,000	20,200,000	20,900,000
Troubled debt restructurings, previous charge-offs			4,400,000	1,700,000	2,100,000
Allowance for loan losses allocated to troubled debt restructurings	2,200,000		2,200,000	1,700,000	2,100,000
Commitments to lend additional funds	0		0
Troubled debt restructurings principal balances	763,000	528,000	2,400,000		1,300,000
Allowance for loan losses adjustments	49,000		203,000
Troubled debt restructurings, provision for loan loss expense			685,000	1,300,000	1,500,000
Purchased Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	17,007,000		17,007,000		6,659,000
Modified loans	8,001,000		8,001,000		12,903,000
Troubled debt restructurings	583,000		583,000		0
Troubled debt restructurings, provision for loan loss expense			4,000
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	39,283,000		39,283,000	74,911,000	69,152,000
Modified loans	10,996,000		10,996,000	13,544,000	20,990,000
Troubled debt restructurings	25,000,000		25,000,000	28,400,000	27,300,000
Troubled debt restructurings, previous charge-offs			2,100,000	3,700,000	1,600,000
Commitments to lend additional funds	0		0
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	100,000		100,000
Loan-to-value ratio	110.00%		110.00%
Minimum [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	200,000		200,000
Maximum [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		1,000,000
Maximum [Member] | Independent Third Party Loan Review [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan relationship with risk ratings reviewed annually	$ 250,000,000		$ 250,000,000