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Allowance for Loan Losses - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 Contract | Dec. 31, 2013 Contract | Dec. 31, 2012 Contracts
Receivables [Abstract] |  |  |
Troubled debt restructurings (TDRs), amount | $ 1,900,000 us-gaap_FinancingReceivableModificationsRecordedInvestment | $ 1,900,000 us-gaap_FinancingReceivableModificationsRecordedInvestment |
TDRs performing under the restructured terms | 790,000 fxnc_PerformingTroubledDebtRestructurings | 829,000 fxnc_PerformingTroubledDebtRestructurings |
Loans modified under TDRs | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 | 0 us-gaap_FinancingReceivableModificationsNumberOfContracts2 | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2
Troubled debt restructuring loan subsequent default | 0 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 | 0 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 | 0 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Troubled debt restructuring loan subsequent default period | 90 days |  |
Non-accrual loans excluded from impaired loan disclosure | 0 fxnc_FinancingReceivableNonAccrualLoansExcludedFromImpairedLoanDisclosure | 14,000 fxnc_FinancingReceivableNonAccrualLoansExcludedFromImpairedLoanDisclosure | 13,000 fxnc_FinancingReceivableNonAccrualLoansExcludedFromImpairedLoanDisclosure
Interest accrued on loan | $ 423,000 us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans | $ 483,000 us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans | $ 510,000 us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans

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- Definition Financing receivable modification as troubled debt restructuring that subsequently defaulted. + References No definition available. + Details
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- Definition Financing receivable non-accrual loans excluded from impaired loan disclosure. + References No definition available. + Details
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- Definition Performing troubled debt restructurings. + References No definition available. + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554 + Details
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- Definition Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524 + Details
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- Definition Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554 + Details
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- Details
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