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Investments (Narrative) (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 | Mar. 31, 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 |  |
Percent of average daily trading volume | 0.20% |  | 0.20%
Net gains on sale of investment securities | $ 4,276,000 | $ 0 |
Carrying value of securities pledged | 311,000,000 |  | $ 337,000,000
Debt Securities [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Credit-related investment impairment losses recognized in earnings | 2,100,000 |  |
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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- Definition The fair value of available-for-sale securities that serve as collateral for borrowings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5086-111524 + Details
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