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Securities - Additional Information (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
Security
Dec. 31, 2016
USD ($)
Security
Dec. 31, 2015
USD ($)
Schedule Of Investments [Line Items]      
Trading securities $ 0 $ 0  
Net gains or (losses) reclassified from accumulated other comprehensive income (loss) into earnings 8,000 3,200,000 $ 3,100,000
Carrying amount of securities pledged as collateral $ 262,500,000 $ 178,600,000  
Minimum      
Schedule Of Investments [Line Items]      
Percentage of shareholders equity for which securities holdings exceeds for no issuer other than U.S. Government and agencies 10.00% 10.00%  
Federal Home Loan Bank Advances      
Schedule Of Investments [Line Items]      
Carrying amount of securities pledged as collateral $ 128,400,000 $ 58,700,000  
Public Fund Deposits      
Schedule Of Investments [Line Items]      
Carrying amount of securities pledged as collateral 99,300,000 93,200,000  
Customer Repurchase Agreements      
Schedule Of Investments [Line Items]      
Carrying amount of securities pledged as collateral $ 34,800,000 $ 19,900,000  
Available-for-sale Securities      
Schedule Of Investments [Line Items]      
Investment securities with unrealized losses | Security 101 88  
Held-to-maturity Securities      
Schedule Of Investments [Line Items]      
Investment securities with unrealized losses | Security 28 42