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Derivatives and Hedging Activities - Narrative (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2019
USD ($)
swap
Dec. 31, 2018
USD ($)
swap
Sep. 30, 2017
USD ($)
Derivative [Line Items]        
Accrued interest on derivative, at fair value   $ 344,000 $ 65,000  
Net liability position   17,500,000    
Collateral against obligations   18,400,000    
Interest rate swaps        
Derivative [Line Items]        
Derivative gains (losses) recorded in the Statements of Income $ 0 204,000 (52,000) $ 0
Federal Home Loan Bank advances | Interest rate swaps        
Derivative [Line Items]        
Notional amount of derivative   410,000,000.0 320,000,000.0  
Not-designated hedge | Currency forward contract - non-designated hedge        
Derivative [Line Items]        
Notional amount of derivative   0 0  
Not-designated hedge | Interest rate swaps        
Derivative [Line Items]        
Notional amount of derivative   $ 169,900,000 $ 36,600,000  
Number of commercial banking customers | swap   22 3  
Designated as hedging instrument | Federal Home Loan Bank advances | Interest rate swaps        
Derivative [Line Items]        
Notional amount of derivative   $ 410,000,000.0 $ 320,000,000.0  
Number of interest rate derivatives held | swap   29 24