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Investments (Narrative) (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Schedule of Investments [Line Items]          
Credit-related net investment impairment losses     $ 0 $ 0 $ 500,000.0
Credit-related investment impairment losses recognized in earnings $ 3,700,000   $ 3,700,000    
Percent of average daily trading volume 0.40%   0.40%   0.40%
Net gains on sale of investment securities $ 0 $ 2,668,000 $ 4,276,000 $ 3,513,000  
Carrying value of securities pledged 364,000,000   364,000,000   $ 337,000,000
Debt Securities          
Schedule of Investments [Line Items]          
Credit-related investment impairment losses recognized in earnings 2,100,000   2,100,000    
Equity Securities          
Schedule of Investments [Line Items]          
Credit-related investment impairment losses recognized in earnings $ 1,600,000   $ 1,600,000    
Maximum | First National Corporation (FXNC)          
Schedule of Investments [Line Items]          
Equity ownership positions in the community bank holding companies 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 1.50%   1.50%   1.50%