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Borrowings (Schedule of Advances to the Federal Home Loan Bank) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Maturity Schedule Of Federal Home Loan Bank Advances [Line Items]    
FHLB advances, carrying value of residential real estate and commercial loans pledged as collateral $ 1,100,000  
FHLB advances, securities pledged as collateral 215 $ 400
Long-term Debt    
Maturity Schedule Of Federal Home Loan Bank Advances [Line Items]    
Long-term Federal Home Loan Bank Advances $ 10,000  
Long term borrowings interest rate (as a percent) 1.87%  
Short Term Borrowing Maturing January 2017 [Member] | Short-term Debt    
Maturity Schedule Of Federal Home Loan Bank Advances [Line Items]    
FHLBB borrowings $ 200,000  
Short Term Borrowing Maturing January 2017 [Member] | Short-term Debt | Minimum [Member]    
Maturity Schedule Of Federal Home Loan Bank Advances [Line Items]    
Federal Home Loan Bank Advances Maturities Summary Fixed Interest Rate Within One Year From Balance Sheet Date 1.45%  
Short Term Borrowing Maturing January 2017 [Member] | Short-term Debt | Maximum [Member]    
Maturity Schedule Of Federal Home Loan Bank Advances [Line Items]    
Federal Home Loan Bank Advances Maturities Summary Fixed Interest Rate Within One Year From Balance Sheet Date 1.63%  
Federal Home Loan Bank, Advances, Callable Option [Member] | Short-term Debt    
Maturity Schedule Of Federal Home Loan Bank Advances [Line Items]    
FHLBB borrowings $ 50,000  
Federal Home Loan Bank Advances Maturities Summary Fixed Interest Rate Within One Year From Balance Sheet Date 1.54%