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Goodwill, Core Deposit Intangibles and Other Intangible Assets (Mortgage and Commercial Loans Servicing Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation servicing rights in tranches [Line Items]        
Mortgage loans serviced for others   $ 3,560,000,000 $ 4,310,000,000  
Custodial escrow balances maintained   17,300,000 21,400,000  
Servicing rights, valuation allowance   0 0  
Unpaid principal balance $ 773,900,000      
Mortgage servicing rights   6,900,000    
Cash received from sale of mortgage servicing rights   6,300,000    
Estimated loss on sale of portfolio   223,000    
Mortgage servicing rights receivable   427,000    
Servicing rights, net   25,857,000 35,778,000  
Commercial Servicing Rights less than 20 Years        
Valuation servicing rights in tranches [Line Items]        
Fair value of mortgage servicing rights   $ 740,000 1,200,000  
Servicing rights, term   20 years    
Servicing rights, valuation allowance   $ 0 0  
Commercial Servicing Rights more than 20 Years        
Valuation servicing rights in tranches [Line Items]        
Fair value of mortgage servicing rights   $ 2,481,000 2,927,000  
Servicing rights, term   20 years    
Servicing rights, valuation allowance   $ 12,000 33,000  
Mortgage Servicing Rights        
Valuation servicing rights in tranches [Line Items]        
Fair value of mortgage servicing rights   37,081,000 45,210,000  
Mortgage servicing rights   $ 6,940,000 $ 0  
Prepayment rate (as a percent)   9.73% 9.63%  
Discount rate (as a percent)   9.06% 9.26%  
Fees collected for servicing of mortgage loans   $ 11,600,000 $ 12,100,000 $ 10,700,000
Mortgage Servicing Rights | Minimum        
Valuation servicing rights in tranches [Line Items]        
Capitalization rate (as a percent)   0.91% 0.88%  
Mortgage Servicing Rights | Maximum        
Valuation servicing rights in tranches [Line Items]        
Capitalization rate (as a percent)   1.50% 1.50%  
Mortgage Servicing Rights 15-year Tranche        
Valuation servicing rights in tranches [Line Items]        
Servicing rights, term   15 years    
Mortgage Servicing Rights 30-year Tranche        
Valuation servicing rights in tranches [Line Items]        
Servicing rights, term   30 years    
Commercial Servicing Rights        
Valuation servicing rights in tranches [Line Items]        
Fair value of mortgage servicing rights   $ 3,221,000 $ 4,127,000  
Fees collected for servicing of mortgage loans   1,700,000 1,900,000  
Servicing rights, net   $ 139,900,000 $ 164,600,000  
Commercial Servicing Rights | Minimum        
Valuation servicing rights in tranches [Line Items]        
Prepayment rate (as a percent)   7.27% 6.96%  
Discount rate (as a percent)   13.04% 12.44%  
Capitalization rate (as a percent)   3.10% 3.10%  
Commercial Servicing Rights | Maximum        
Valuation servicing rights in tranches [Line Items]        
Prepayment rate (as a percent)   8.88% 7.88%  
Discount rate (as a percent)   15.49% 13.88%  
Capitalization rate (as a percent)   4.45% 4.45%