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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%

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