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Loans - Impaired Loans Narrative (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Debt Instrument [Line Items]      
Carrying amount of loan investments $ 3,120,543 $ 2,860,921 $ 2,822,555
Impaired loans 12,300    
Recorded investment of impaired loans 28,459 38,282  
Collective related allowance for loan losses for impaired loans 1,112 2,408  
Non Accrual [Member]      
Debt Instrument [Line Items]      
Investment in impaired loans not covered by loan loss, non-accrual status 19,200    
Fair Value Of Collateral Pledged [Member]      
Debt Instrument [Line Items]      
Impaired loans 17,400    
Discounted Cash Flows [Member]      
Debt Instrument [Line Items]      
Impaired loans 2,500    
General Reserve [Member]      
Debt Instrument [Line Items]      
Impaired loans 8,600    
Individual loan amount of impaired loans 250    
Commercial Loan [Member]      
Debt Instrument [Line Items]      
Carrying amount of loan investments 1,807,883 $ 1,560,430  
Commercial and Industrial [Member]      
Debt Instrument [Line Items]      
Impaired loans 3,000    
Energy Loans [Member]      
Debt Instrument [Line Items]      
Impaired loans 6,000    
Agricultural Sector [Member]      
Debt Instrument [Line Items]      
Impaired loans 1,300    
Owner-Occupied [Member]      
Debt Instrument [Line Items]      
Impaired loans $ 2,000