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Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Period of maturity as classifies as short-term borrowings	90 days
Fair value of loan commitments and standby letters of credit	$ 0	$ 0
Loans Held For Sale [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Non recurring fair value adjustments on loans held for sale	$ 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

X

- Definition

Fair value of loan commitments and standby letters of credit.

+ References

No definition available.

+ Details

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

X

- Definition

Period of appraisal of property to consider different method to measure fair value.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Definition

Period of maturity as classifies as short term borrowing.

+ References

No definition available.

+ Details

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

X

- Definition

Period used to determine value of business equipment.

+ References

No definition available.

+ Details

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

X

- Definition

Maximum period used to determine value of real estate collateral.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Amount of addition (reduction) to the amount at which an asset could be incurred (settled) in a current transaction between willing parties.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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X

- Details

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X

- Details

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- Details

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- Details

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