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Borrowings (Narrative) (Details) - USD ($)
 12 Months Ended
Apr. 15, 2015
Dec. 31, 2017
Dec. 31, 2015
Dec. 31, 2016
Jun. 30, 2015
Short-term Debt [Line Items]          
Short-term borrowings, weighted average rate   1.50%   0.76%  
Short-term borrowings   $ 446,200,000   $ 331,500,000  
Debt issuance costs     $ 1,060,000    
Subordinated Notes Due April 15, 2030 [Member]          
Short-term Debt [Line Items]          
Proceeds from issuance of debt, net   38,900,000      
Debt issuance costs   $ 1,100,000      
Principal amount $ 40,000,000       $ 40,000,000
Interest rate 6.00%        
Number of years at stated rate   10 years      
Debt issuance cost amortization period   15 years      
Federal Home Loan Bank Advances [Member]          
Short-term Debt [Line Items]          
Short-term borrowings   $ 446,200,000   331,500,000  
Federal Home Loan Bank Overnight Borrowings [Member]          
Short-term Debt [Line Items]          
Short-term borrowings   304,700,000   171,500,000  
Federal Home Loan Borrowings Short Term Advances [Member]          
Short-term Debt [Line Items]          
Short-term borrowings   $ 141,500,000   $ 160,000,000  
LIBOR [Member] | Subordinated Notes Due April 15, 2030 [Member]          
Short-term Debt [Line Items]          
Spread on variable rate 3.944%