Funds Restrictions and Reserve Balance - Additional Information (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Aggregate amount of unrestricted funds	$ 253,000
First Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Aggregate amounts of daily average required balances of reserve against its deposits	$ 1,400,000	$ 3,000,000

Aggregate amount of unrestricted funds transferred from bank.

No definition available.

Name:	fxnc_AggregateAmountOfUnrestrictedFundsTransferredFromBank
Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant