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Note 7 - Capital Requirements (Details Textual) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2018
Capital Conservation Buffer Phase, Period | 4 years |  |
Capital Conservation Buffer Percentage, Annual Installment | 0.625% |  |
Capital Conservation Buffer, Percentage | 2.50% |  | 1.875%
Capital Conservation Buffer Percentage, Maintained at Bank |  | 6.24% | 5.62%

X
- Definition Represents the required amount of capital conservation buffer as a percentage. + References No definition available. + Details
Name: | fxnc_CapitalConservationBufferPercentage
Namespace Prefix: | fxnc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Represents the amount of capital conservation buffer, as a percentage, required to be contributed per year over the capital conservation phase-in period. + References No definition available. + Details
Name: | fxnc_CapitalConservationBufferPercentageAnnualInstallment
Namespace Prefix: | fxnc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the amount of capital conservation buffer, as a percentage, maintained at a bank. + References No definition available. + Details
Name: | fxnc_CapitalConservationBufferPercentageMaintainedAtBank
Namespace Prefix: | fxnc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The length of time in which the capital conservation buffer will be phased in time. + References No definition available. + Details
Name: | fxnc_CapitalConservationBufferPhasePeriod
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration