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Investments (Narrative) (Details) - USD ($)
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Schedule of Investments [Line Items]      
Federal Home Loan Bank Stock and Federal Reserve Bank Stock $ 16,400,000   $ 18,500,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, Additional Credit Losses $ 0 $ 0  
Percent of average daily trading volume 0.20% 0.20% 0.20%
Carrying Value of Securities Pledged $ 481,000,000   $ 510,000,000
Non-Governmental Issues Exceeding 10% Shareholders Equity Threshold 0   0
Equity Securities      
Schedule of Investments [Line Items]      
Equity Securities, FV-NI $ 11,700,000   $ 11,800,000
Maximum | First National Corporation (FXNC)      
Schedule of Investments [Line Items]      
Equity ownership positions in the community bank holding companies 4.00%   4.00%
Maximum | Eagle Financial Services, Inc. (EFSI)      
Schedule of Investments [Line Items]      
Equity ownership positions in the community bank holding companies 1.50%   1.50%