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)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
SecurityLoan
Sep. 30, 2017
USD ($)
Property
Dec. 31, 2016
USD ($)
Property
Dec. 31, 2015
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Accrued interest receivable   $ 12,098 $ 10,987  
Loans and leases receivable   $ 4,162,863 $ 3,812,340  
Number of real estate properties | Property   6 12  
Other real estate owned (OREO), net   $ 1,946 $ 7,484  
TDR loans receivable   $ 15,600 18,000  
Principal and interest due   3 months    
Reserves relating to loans included in allowance for loan losses   $ 2,200 3,400  
PCI Loans and Leases [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loans and leases receivable $ 10,007 $ 7,910 8,892 $ 14,573
Central Bancorp, Inc [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Number of real estate properties | Property   5    
Other real estate owned (OREO), net   $ 1,800 5,700  
Securities Pledged as Collateral [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Loans and leases receivable   1,100,000 1,000,000  
Loans and Finance Receivables [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Accrued interest receivable   $ 9,500 $ 8,200  
Commercial term [Member] | Commercial and industrial loans [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Number of Loans | SecurityLoan 1      
Recorded Investment $ 53