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Investments (Narrative) (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Schedule of Investments [Line Items]          
Federal Home Loan Bank Stock and Federal Reserve Bank Stock $ 15,600,000   $ 15,600,000   $ 15,900,000
Non-governmental issues exceeding 10% shareholders equity threshold 0   0   0
Held-to-maturity securities in an unrealized loss position         0
Credit-related investment impairment losses     0 $ 0  
Sale of VISA shares 0 $ 0 17,837,000 $ 0  
Asset Pledged as Collateral          
Schedule of Investments [Line Items]          
Carrying value of securities pledged 618,000,000   618,000,000   519,000,000
Marketable equity securities          
Schedule of Investments [Line Items]          
Investment securities $ 10,900,000   10,900,000   12,600,000
VISA, Inc. (VISA)          
Schedule of Investments [Line Items]          
Sale of VISA shares     $ 17,800,000    
VISA, Inc. (VISA) | Common Class B          
Schedule of Investments [Line Items]          
Shares of Visa Inc. Class B common shares sold (in shares) 86,605   86,605    
VISA, Inc. (VISA) | Marketable equity securities          
Schedule of Investments [Line Items]          
Investment securities         $ 0
Maximum | First National Corporation (FXNC)          
Schedule of Investments [Line Items]          
Equity ownership positions in the community bank holding companies (percent) 4.00%   4.00%   4.00%
Maximum | Eagle Financial Services, Inc. (EFSI)          
Schedule of Investments [Line Items]          
Equity ownership positions in the community bank holding companies (percent) 1.50%   1.50%   1.50%