Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	May. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying amount of loans	$ 7,692,000
Non-accrual loans	37,649,000				$ 27,382,000
Loans modified as troubled debt restructurings	27,000,000				9,300,000
Loans 90 days past due and accruing	7,645,000				3,830,000
Proceeds from sales of loans held for sale	58,054,000	$ 31,038,000
Allowance for loan losses	77,721,000	$ 76,802,000		$ 77,479,000	$ 76,140,000	$ 75,514,000	$ 74,751,000
Commitments to lend to borrowers with loan modifications classified as TDR's	293,000
Default payment of troubled restructuring, commercial real estate loan	178,000
PCI loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans modified as troubled debt restructurings	0
Loans 90 days past due and accruing	5,023,000
Allowance for loan losses	0
Loans Accounted for under ASC 310-30 [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying amount of loans	7,700,000
Outstanding balance of loans	9,100,000
Marquette [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans purchased at fair value			$ 980,300,000
Marquette [Member] | Non-PCI loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans purchased at fair value			972,500,000
Contractually required principal and interest at acquisition	983,900,000
Marquette [Member] | PCI loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans purchased at fair value			$ 7,800,000
Contractually required principal and interest at acquisition	$ 9,300,000
Revolving Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Number of days past due to consider loan as a loss and charged off	90 days
Closed-End Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Number of days past due to consider loan as a loss and charged off	120 days
Open-End Retail Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Number of days past due to consider loan as a loss and charged off	180 days