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Investments (Narrative) (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Schedule of Investments [Line Items]          
Credit-related net investment impairment losses     $ 0 $ 0  
Percent of average daily trading volume 0.50%   0.50%   0.40%
Unrealized gains recognized on securities still held $ 592,000   $ 772,000    
Reclassification from AOCI to retained earnings     0    
Net gains on sale of investment securities 0 $ 0 0 $ 4,276,000  
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%
Accumulated Other Comprehensive Income (Loss)          
Schedule of Investments [Line Items]          
Reclassification from AOCI to retained earnings     $ 2,657,000    
Accounting Standards Update 2016-01          
Schedule of Investments [Line Items]          
Reclassification from AOCI to retained earnings     2,700,000    
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income (Loss)          
Schedule of Investments [Line Items]          
Reclassification from AOCI to retained earnings     2,657,000    
Asset Pledged as Collateral [Member]          
Schedule of Investments [Line Items]          
Debt Securities, Available-for-sale, Restricted $ 472,000,000   $ 472,000,000   $ 429,000,000