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Loans - Narrative (Details)
 9 Months Ended
Sep. 30, 2017
USD ($)
loan
Sep. 30, 2016
USD ($)
loan
Dec. 31, 2016
USD ($)
loan
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Number of loans over 90 days past due and still accruing interest | loan 0   0
Unadvanced commitments on impaired loans     $ 0
Specific reserves on restructured loans $ 2,200,000.0    
Restructuring Reserve 9,500,000    
Unadvanced Commitment on Impaired Loan      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Estimated losses attributable to unadvanced commitments on impaired loans 400,000   300,000
Non-Covered Loans      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Post-Modification Outstanding Recorded Balance $ 676,000 $ 2,609,000  
Financing Receivable, Modifications, Number of Contracts | loan 1 4  
Restructuring Reserve $ 721,000   2,331,000
Financing Receivable, Gross $ 3,949,856,000   3,119,154,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | loan 3    
Non-Covered Loans | Consumer and other      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Post-Modification Outstanding Recorded Balance $ 0 $ 0  
Financing Receivable, Modifications, Number of Contracts | loan 0 0  
Restructuring Reserve $ 0   0
Financing Receivable, Gross $ 155,514,000   $ 156,182,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | loan 0