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Derivatives and Hedging Activities - Narrative (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
swap
Dec. 31, 2018
USD ($)
swap
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Derivative [Line Items]        
Accrued interest on derivatives, at fair value $ (7) $ (65)    
Net liability position   2,600    
Collateral against obligations   2,600    
Interest rate products-designated        
Derivative [Line Items]        
Derivative gains (losses) recorded in the Statements of Income 0 52 $ 0 $ 0
Federal Home Loan Bank advances | Interest rate products-designated        
Derivative [Line Items]        
Notional amount of derivative 20,000 320,000    
Not-designated hedge | Currency forward contract - non-designated hedge        
Derivative [Line Items]        
Notional amount of derivative 1,600 0    
Not-designated hedge | Interest rate products-designated        
Derivative [Line Items]        
Notional amount of derivative 0 $ 36,600    
Number of swaps with customers offset by third party swaps | swap   3    
Designated as hedging instrument | Federal Home Loan Bank advances | Interest rate products-designated        
Derivative [Line Items]        
Notional amount of derivative $ 20,000 $ 320,000    
Number of interest rate derivatives held | swap 2 24