Quarterly report pursuant to Section 13 or 15(d)

Loans and Leases - Additional Information (Detail)

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Loans and Leases - Additional Information (Detail) - USD ($)
 3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Total gross loans and leases receivable $ 4,413,557,000 $ 4,304,458,000
Accrued interest receivable 12,751,000 12,770,000
Loans and leases receivable 4,381,780,000 4,273,415,000
Loans 90 days or more past due and still accruing 17,000 0
Non-accrual Non-PCI loans and leases 15,345,000 15,805,000
Other real estate owned (OREO), net 1,660,000 1,946,000
TDR loans receivable $ 14,600,000 15,400,000
Principal and interest due 3 months  
Reserves relating to loans included in allowance for loan losses $ 1,700,000 2,200,000
Home equity line of credit [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Total gross loans and leases receivable 12,700,000 14,200,000
Securities Pledged as Collateral [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Loans and leases receivable 1,100,000,000 1,100,000,000
Loans and Finance Receivables [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accrued interest receivable 10,300,000 10,200,000
Non-Accrual Loans and Leases, Excluding PCI Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Non-accrual Non-PCI loans and leases $ 1,000,000 $ 900,000