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Deposits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deposits [Abstract]
Time deposits, in denominations of $250 thousand or more	$ 11.4fxnc_TimeDeposits250000OrMore	$ 11.8fxnc_TimeDeposits250000OrMore
Brokered deposits	$ 1.9fxnc_BrokeredDeposits	$ 5.1fxnc_BrokeredDeposits

X

- Definition

Total brokered deposits.

+ References

No definition available.

+ Details

Name:	fxnc_BrokeredDeposits
Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Time Deposits 250000 Or More

+ References

No definition available.

+ Details

Name:	fxnc_TimeDeposits250000OrMore
Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_DepositsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration