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Borrowings - Lines of Credit (Details) - Line of credit - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Federal Funds Purchased    
Line of Credit Facility [Line Items]    
Amount of borrowings outstanding $ 0 $ 0
Federal Home Loan Bank Borrowings    
Line of Credit Facility [Line Items]    
Lines of credit 616,200,000 538,900,000
Federal Home Loan Bank Overnight Borrowings    
Line of Credit Facility [Line Items]    
Amount of borrowings outstanding 0 0
Federal Reserve Line of Credit    
Line of Credit Facility [Line Items]    
Lines of credit 64,400,000 52,100,000
Amount of borrowings outstanding 0 0
Unsecured Debt | Federal Funds Purchased    
Line of Credit Facility [Line Items]    
Lines of credit $ 92,000,000 $ 100,400,000