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Transactions with Related Parties - Additional Information (Detail) (First Bank [Member], USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
First Bank [Member]
   
Related Party Transaction [Line Items]    
Total loans $ 2,400,000us-gaap_LoansAndLeasesReceivableRelatedParties
/ dei_LegalEntityAxis
= fxnc_FirstBankMember
$ 2,100,000us-gaap_LoansAndLeasesReceivableRelatedParties
/ dei_LegalEntityAxis
= fxnc_FirstBankMember
Total principal additions 496,000us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions
/ dei_LegalEntityAxis
= fxnc_FirstBankMember
 
Total principal payments 164,000us-gaap_LoansAndLeasesReceivableRelatedPartiesCollections
/ dei_LegalEntityAxis
= fxnc_FirstBankMember
 
Deposits from related parties held by Bank $ 5,700,000fxnc_DepositRelatedParty
/ dei_LegalEntityAxis
= fxnc_FirstBankMember
$ 5,100,000fxnc_DepositRelatedParty
/ dei_LegalEntityAxis
= fxnc_FirstBankMember