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Note 20 - Regulatory Matters (Details Textual) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Capital Conservation Buffer, Percentage | 2.50% | 2.50%
Capital Conservation Buffer Percentage, Maintained at Bank | 7.82% | 6.84%

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, 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