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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

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