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Goodwill, Core Deposit Premium and Other Intangible Assets (Mortgage and commercial loans servicing) (Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Sep. 30, 2017
Mar. 31, 2017
Dec. 31, 2017
Valuation servicing rights in tranches [Line Items]        
Mortgage loans serviced for others $ 3,540,000,000     $ 3,560,000,000
Custodial escrow balances maintained 23,200,000     17,300,000
Total net loans receivable held to maturity   $ 773,900,000    
Amount derecognized 6,900,000      
Loans serviced 25,471,000     25,857,000
Mortgage Servicing Rights        
Valuation servicing rights in tranches [Line Items]        
Servicing asset at amortized value $ 40,434,000   $ 47,564,000 $ 37,100,000
Prepayment rate (percent) 8.23%     9.73%
Discount rate (percent) 9.04%     9.06%
Fees collected for servicing of mortgage loans $ 2,200,000   $ 3,200,000  
Mortgage Servicing Rights | Minimum        
Valuation servicing rights in tranches [Line Items]        
Average cap rate (percent) 0.96%   0.91%  
Mortgage Servicing Rights | Maximum        
Valuation servicing rights in tranches [Line Items]        
Average cap rate (percent) 1.25%   1.50%  
Mortgage Servicing Rights 15-year Tranche        
Valuation servicing rights in tranches [Line Items]        
Servicing asset, agreement term 15 years      
Mortgage Servicing Rights 30-year Tranche        
Valuation servicing rights in tranches [Line Items]        
Servicing asset, agreement term 30 years      
Commercial Servicing Rights        
Valuation servicing rights in tranches [Line Items]        
Servicing asset at amortized value $ 2,781,000   $ 4,040,000 $ 3,200,000
Loans serviced $ 125,500,000     $ 139,900,000
Commercial Servicing Rights | Minimum        
Valuation servicing rights in tranches [Line Items]        
Prepayment rate (percent) 8.27%     7.27%
Discount rate (percent) 13.09%     13.04%
Average cap rate (percent) 3.10%   3.10%  
Commercial Servicing Rights | Maximum        
Valuation servicing rights in tranches [Line Items]        
Prepayment rate (percent) 9.89%     8.88%
Discount rate (percent) 16.71%     15.49%
Average cap rate (percent) 4.45%   4.45%  
Commercial Servicing Rights Less Than 20 Years        
Valuation servicing rights in tranches [Line Items]        
Servicing asset at amortized value $ 648,000     $ 740,000
Servicing asset, agreement term 20 years      
Fees collected for servicing of mortgage loans $ 420,000   $ 415,000  
Valuation allowance 0     0
Commercial Servicing Rights More Than 20 Years        
Valuation servicing rights in tranches [Line Items]        
Servicing asset at amortized value $ 2,133,000     2,481,000
Servicing asset, agreement term 20 years      
Valuation allowance $ 14,000     $ 12,000