Allowance For Loan Losses [Line Items]

No definition available.

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- Definition

Financing receivable modification as troubled debt restructuring that subsequently defaulted.

+ References

No definition available.

+ Details

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Namespace Prefix:	fxnc_
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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

+ Details

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- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Data Type:	na
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Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($) Contract	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($) Contract	Dec. 31, 2014 USD ($)
Allowance For Loan Losses [Line Items]
TDRs performing under the restructured terms	$ 1,400,000		$ 1,400,000		$ 1,900,000
Loans modified under TDRs \| Contract	0	0	0	1
Pre-modification recorded investment totaled				$ 283,000
Post-modification recorded investment totaled				344,000
Troubled debt restructuring loan subsequent default	$ 0	$ 0	$ 0	$ 0
Troubled debt restructuring loan subsequent default period			90 days
Performing Financing Receivable [Member]
Allowance For Loan Losses [Line Items]
TDRs performing under the restructured terms	$ 321,000		$ 321,000		$ 790,000