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Note 2 - Securities - Investments in an Unrealized Loss Position That Were Temporarily Impaired (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Mar. 31, 2020
--- | --- | ---
Less than 12 months, fair value, available for sale securities | $ 59,111 | $ 18,387
Less than 12 months, unrealized (loss), available for sale securities | (1,238) | (82)
12 months or more, fair value, available for sale securities |  |
12 months or more, unrealized (loss), available for sale securities |  |
Total, fair value, available for sale securities | 59,111 | 18,387
Total, unrealized (loss), available for sale securities | (1,238) | (82)
US Agency and Mortgage-backed Securities [Member] |  |
Less than 12 months, fair value, available for sale securities | 37,512 | 17,367
Less than 12 months, unrealized (loss), available for sale securities | (795) | (80)
12 months or more, fair value, available for sale securities |  |
12 months or more, unrealized (loss), available for sale securities |  |
Total, fair value, available for sale securities | 37,512 | 17,367
Total, unrealized (loss), available for sale securities | (795) | (80)
Municipal Bonds [Member] |  |
Less than 12 months, fair value, available for sale securities | 21,599 | 1,020
Less than 12 months, unrealized (loss), available for sale securities | (443) | (2)
12 months or more, fair value, available for sale securities |  |
12 months or more, unrealized (loss), available for sale securities |  |
Total, fair value, available for sale securities | 21,599 | 1,020
Total, unrealized (loss), available for sale securities | $ (443) | $ (2)

X
- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_InvestmentTypeAxis=fxnc_USAgencyAndMortgagebackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_MunicipalBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |