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Benefit Plans - Additional Information (Detail) (USD $) | 0 Months Ended | 12 Months Ended
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May 31, 2013 | Jan. 01, 2000 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Number of highest-paid consecutive years for benefits |  |  | 5 years |  |  |
Percentage of first employee compensation for which company makes matching contribution |  |  | 1.00%fxnc_DefinedContributionPlanPercentageOfEmployerMatchingContributionOfFirstOnePercentOfEmployeeContribution |  |  |
Company contribution on employee specific contribution |  |  | 50.00%fxnc_DefinedContributionPlanPercentageOfEmployerMatchingContributionOfStatedPercentageOfEmployeeContribution |  |  |
Number of hours in service required for the contribution to be allocated |  |  | 1000 hours |  |  |
Minimum eligibility for employees under 401(k) Plan, age |  |  | 19 years |  |  |
Number of plan service years after which employer matching contribution vest |  |  | 2 years |  |  |
Expense attributable to the Plan amounted |  |  | $ 306,000 us-gaap_DefinedContributionPlanCostRecognized | $ 249,000 us-gaap_DefinedContributionPlanCostRecognized | $ 219,000 us-gaap_DefinedContributionPlanCostRecognized |
Percentage of vested participants |  | 100.00%fxnc_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingPercentageUnderAwardRequisiteServicePeriod |  |  |  |
Participants years of credited service |  | 2 years |  |  |  |
Compensation expense for ESOP |  |  | 0 us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense | 0 us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense | 0 us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense |
Shares of the Company held by the ESOP |  |  | 173,823 us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP | 169,882 us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP | 134,609 us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP |
Recorded gain from termination of post retirement benefit liability | 543,000 us-gaap_DefinedBenefitPlanOtherChanges |  |  |  |  |
Minimum [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Percentage of employee for which company makes specified contribution |  |  | 2.00%fxnc_DefinedContributionPlanStatedPercentageOfEmployeesContribution / us-gaap_RangeAxis = us-gaap_MinimumMember |  |  |
Maximum [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Percentage of employee for which company makes specified contribution |  |  | 6.00%fxnc_DefinedContributionPlanStatedPercentageOfEmployeesContribution / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |
Postretirement Life Insurance [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Related effect on net income recognized expense (benefit) |  |  |  |  | 21,000 fxnc_ExpenseBenefitRecognizedUnderPlan / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedBenefitPostretirementLifeInsuranceMember |
Defined Benefit Pension Plan [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Minimum eligibility for full-time employees, age |  |  | 21 years |  |  |
Effective discount rate |  |  | 0.25%fxnc_DefinedBenefitPlanSingleEffectiveDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  |
Amount contributed to pension plan |  |  | 0 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 500,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 306,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Amount of accumulated benefit obligation for the defined benefit pension plan |  |  | 5,300,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 3,800,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 5,100,000 us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Defined Benefit Pension Plan [Member] | Scenario, Forecast [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Amount contributed to pension plan, Expected in future |  |  |  |  |  | $ 0 us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Defined Benefit Pension Plan [Member] | Minimum [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Period of service credit by employee |  |  | 1 year |  |  |
Defined Benefit Pension Plan [Member] | Fixed Income Funds [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Targeted asset allocation on fixed income |  |  | 40.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  |
Defined Benefit Pension Plan [Member] | Equity Funds [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Targeted asset allocation on fixed income |  |  | 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  |  |

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- Definition Defined benefit pension plan minimum eligible age of employee. + References No definition available. + Details
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- Definition Defined benefit plan single effective discount rate. + References No definition available. + Details
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- Definition Defined contribution plan minimum eligible age of employee. + References No definition available. + Details
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- Definition Defined contribution plan percentage of employer matching contribution of first one percent of employee contribution. + References No definition available. + Details
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- Definition Defined contribution plan percentage of employer matching contribution of stated percentage of employee contribution. + References No definition available. + Details
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- Definition Defined contribution plan stated percentage of employees contribution. + References No definition available. + Details
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- Definition Employee matching contribution vesting period under plan. + References No definition available. + Details
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- Definition Expense benefit recognized under plan. + References No definition available. + Details
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Balance Type: | debit
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- Definition Number of highest paid consecutive periods of service for calculation of average compensation related to benefits. + References No definition available. + Details
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- Definition Period of service credit by employee. + References No definition available. + Details
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- Definition Services completed by eligible participant under plan. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award vesting percentage under award requisite service period. + References No definition available. + Details
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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Details
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- Definition Changes in the defined benefit obligation, not otherwise separately disclosed in the financial statements. + References No definition available. + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan in the current remaining fiscal period. + References No definition available. + Details
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- Definition Target allocation of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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- Definition The amount of plan compensation cost recognized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Definition Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Details
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