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

X

- Definition

Financing receivable modification as troubled debt restructuring that subsequently defaulted.

+ References

No definition available.

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- Definition

Financing receivable non-accrual loans excluded from impaired loan disclosure.

+ References

No definition available.

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- Definition

Performing troubled debt restructurings.

+ References

No definition available.

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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

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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

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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

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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

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