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Acquisition - Additional Information (Detail) (USD $) | 0 Months Ended | 3 Months Ended |  |
--- | --- | --- | --- | ---
Nov. 18, 2014 Branches | Jun. 30, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Business Acquisition [Line Items] |  |  |  |
Total assets after acquisition |  |  | $ 518,165,000 us-gaap_Assets | $ 522,890,000 us-gaap_Assets
Virginia [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of acquired branches | 6 fxnc_NumberOfBranchesAcquired / us-gaap_StatementGeographicalAxis = stpr_VA |  |  |
Scenario, Forecast [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Total assets after acquisition |  | 750,000,000 us-gaap_Assets / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |
Amount for acquisition of deposits |  | 308,000,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |
Amount for acquisition of premises and equipment |  | $ 1,700,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |
Percentage of premium paid on deposit |  | 2.25%fxnc_PercentageOfPremiumPaidOnDeposit / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |

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- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.18) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5 + Details
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- Definition Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition The amount of property, plant, and equipment recognized as of the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=35744172&loc=d3e2207-128464 + Details
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