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Other Borrowings and Notes Payable (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Feb. 23, 2016
Debt Instrument [Line Items]      
Line of Credit, Current     $ 20,000,000
Securities sold under customer repurchase agreements $ 253,674,000 $ 276,980,000  
Total 253,674,000 276,980,000  
Amount withdrawn 0 0  
Other Borrowings      
Debt Instrument [Line Items]      
Maximum balance outstanding at any month-end 253,674,000 276,980,000  
Average balance during the year $ 220,807,000 $ 206,643,000  
Average interest rate during the year 0.21% 0.19%  
Average interest rate at December 31 0.25% 0.18%  
Line of Credit      
Debt Instrument [Line Items]      
Maximum balance outstanding at any month-end $ 10,000,000 $ 0  
Average balance during the year $ 822,000 $ 0  
Average interest rate during the year 3.50% 0.00%  
Average interest rate at December 31 0.00% 0.00%  
Remaining borrowing capacity $ 898,100,000    
Aggregate of pledge secured by certain eligible loans 1,071,600,000    
Amount withdrawn $ 0