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Nature of Banking Activities and Significant Accounting Policies - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 Entity | Dec. 31, 2013 | Dec. 31, 2012
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Number of accounts included by parent company | 6 fxnc_NumberOfCompaniesForWhichConsolidatedFinancialStatementsPresented |  |
Recognition of other than temporary impairment on basis of credit loss description | If there is no credit loss, there is no other-than-temporary impairment. If there is a credit loss, other-than-temporary impairment exists, and the credit loss must be recognized in net income and the remaining portion of impairment must be recognized in other comprehensive income. |  |
Gain (loss) on rate lock commitments | $ 0 fxnc_GainLossOnRateLockCommitments |  |
Accrual status of loan | 90 days |  |
Troubled debt restructurings (TDRs), amount | 1,900,000 us-gaap_FinancingReceivableModificationsRecordedInvestment | 1,900,000 us-gaap_FinancingReceivableModificationsRecordedInvestment |
Largest amount of tax benefit realized by taxing authority | 50.00%fxnc_LargestAmountOfTaxBenefitRealizedByTaxingAuthority |  |
Liability for unrecognized tax benefits | 0 us-gaap_UnrecognizedTaxBenefits | 0 us-gaap_UnrecognizedTaxBenefits |
Potential common shares that would have a dilutive effect | 0 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount |  |
Average number of common shares outstanding basic and diluted | 4,902,389 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted | 4,901,464 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted | 3,944,506 us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted
Advertising cost | 313,000 us-gaap_AdvertisingExpense | 269,000 us-gaap_AdvertisingExpense | 364,000 us-gaap_AdvertisingExpense
Minimum [Member] |  |  |
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Period of time between issuance of a loan commitment and closing and sale of the loan | 30 days |  |
Company not considered value of impairment on individual consumer, residential and small commercial loans | 250,000 fxnc_AmountOfSmallBalanceHomogeneousImpairedLoansNotSeparatelyIdentified / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
Management's policy on evaluation of impairment | $ 250,000 fxnc_AmountOfSubstandardAndDoubtfulLoansEvaluatedAsImpaired / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
Minimum [Member] | Software [Member] |  |  |
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of depreciation of premises and equipment | 3 years |  |
Minimum [Member] | Premises and Equipment [Member] |  |  |
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of depreciation of premises and equipment | 3 years |  |
Maximum [Member] |  |  |
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Period of time between issuance of a loan commitment and closing and sale of the loan | 60 days |  |
Maximum [Member] | Software [Member] |  |  |
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of depreciation of premises and equipment | 7 years |  |
Maximum [Member] | Premises and Equipment [Member] |  |  |
Nature Of Banking Activities And Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of depreciation of premises and equipment | 40 years |  |

X
- Definition Amount of small balance homogeneous impaired loans not separately identified. + References No definition available. + Details
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- Definition Gain loss on rate lock commitments. + References No definition available. + Details
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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Antidilution -URI http://asc.fasb.org/extlink&oid=6505113 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Diluted Earnings Per Share -URI http://asc.fasb.org/extlink&oid=6510752 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Contingent Stock Agreement -URI http://asc.fasb.org/extlink&oid=6508534 + Details
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