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Investments (Narrative) (Details) - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015
Schedule of Investments [Line Items] |  |  |
Credit-related net investment impairment losses | $ 400,000.0 | $ 0 | $ 0
Credit-related investment impairment losses recognized in earnings | $ 10,500,000 |  |
Percent of average daily trading volume | 0.20% |  | 0.20%
Carrying value of securities pledged | $ 285,000,000 |  | $ 273,000,000
Debt Securities [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Credit-related investment impairment losses recognized in earnings | 8,900,000 |  |
Equity Securities [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Credit-related investment impairment losses recognized in earnings | $ 1,600,000 |  |
Maximum [Member] | First National Corporation (FXNC) [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Equity ownership positions in the community bank holding companies | 4.00% |  | 4.00%
Maximum [Member] | Eagle Financial Services, Inc. (EFSI) [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Equity ownership positions in the community bank holding companies | 1.50% |  | 1.50%

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