Fair Value Measurements - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Non recurring fair value adjustments on loans held for sale	$ 0	$ 0
Period of maturity as classifies as short-term borrowings	90 days
Fair value of loan commitments and standby letters of credit	$ 0	$ 0
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Period under consideration for valuation of real estate collateral	12 months
Period under consideration for valuation of business equipment	1 year
Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Period under consideration for valuation of house or building in the process of construction	1 year

Fair value of loan commitments and standby letters of credit.

No definition available.

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

Non recurring fair value adjustments on loans held for sale.

No definition available.

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Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Minimum period of appraisal of property to consider different method to measure fair value.

No definition available.

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Namespace Prefix:	fxnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Period of maturity as classifies as short term borrowing.

No definition available.

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Namespace Prefix:	fxnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Maximum period used to determine value of business equipment.

No definition available.

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Namespace Prefix:	fxnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Maximum period used to determine value of real estate collateral.

No definition available.

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Namespace Prefix:	fxnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: