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Note 13 - Benefit Plans (Details Textual) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Mar. 15, 2019 | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  | 1.00% |
Defined Contribution Plan, Employer Matching Contribution for Employee's Contribution Between Two and Six Percent, Percent of Match |  | 50.00% |
Defined Contribution Plan, Service Hours Required by Eligible Employees to Receive Additional Contributions |  | 1,000 |
Defined Contribution Plan, Minimum Age for Eligibility (Year) |  | 19 years |
Defined Contribution Plan, Employer Matching Contribution, Vesting Period (Year) |  | 2 years |
The 401(k) Plan [Member] |  |  |
Defined Contribution Plan, Cost |  | $ 815 | $ 812
Supplemental Employee Retirement Plan [Member] |  |  |
Defined Contribution Plan, Cost |  | $ 264 | $ 220
Defined Contribution Plan, Number of Employees | 3 |  |
Defined Contribution Plan, Retirement Benefits Payable, Number of Monthly Payments | 180 |  |
Minimum [Member] |  |  |
Defined Contribution Plan, Stated Percentage of Employees Contribution |  | 2.00% |
Maximum [Member] |  |  |
Defined Contribution Plan, Stated Percentage of Employees Contribution |  | 6.00% |

X
- Definition The percentage the employer matches of the employee's percentage contribution between two and six percent. + References No definition available. + Details
Name: | fxnc_DefinedContributionPlanEmployerMatchingContributionForEmployeesContributionBetweenTwoAndSixPercentPercentOfMatch
Namespace Prefix: | fxnc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The period of time over which the employer's matching contributions vest under the benefit plan. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
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X
- Definition The minimum age an employee must be in order to participate in the benefit plan. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of fixed, monthly payments under the retirement benefit plan. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of hours of service required by eligible employees in order to receive additional contributions from the employer under the benefit plan. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents state percentage of employees contribution for defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

X
- Definition Number of employees eligible to participate in defined contribution plan. + References No definition available. + Details
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X
- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
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Balance Type: |
Period Type: |

X
- Details
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Period Type: |