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Trust Preferred Capital Notes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
First National (VA) Statutory Trust III [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Formation of wholly-owned subsidiary	Jul. 24, 2006
Preferred securities issued through pooled underwriting date	Jul. 31, 2006
Issuance of trust preferred securities	$ 4.0us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustThreeMember
LIBOR-indexed floating rate of interest	1.86%us-gaap_DebtInstrumentInterestRateStatedPercentage / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustThreeMember	1.84%us-gaap_DebtInstrumentInterestRateStatedPercentage / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustThreeMember
Junior subordinated debt securities	4.1us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustThreeMember
Securities mandatory redemption date	Oct. 01, 2036
Maximum capital required for capital adequacy	25.00%fxnc_MaximumCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustThreeMember
First National (VA) Statutory Trust II [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Formation of wholly-owned subsidiary	Jun. 08, 2004
Preferred securities issued through pooled underwriting date	Jun. 17, 2004
Issuance of trust preferred securities	5.0us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustTwoMember
LIBOR-indexed floating rate of interest	2.87%us-gaap_DebtInstrumentInterestRateStatedPercentage / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustTwoMember	2.84%us-gaap_DebtInstrumentInterestRateStatedPercentage / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustTwoMember
Junior subordinated debt securities	$ 5.2us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust / dei_LegalEntityAxis = fxnc_FirstNationalVaStatutoryTrustTwoMember
Securities mandatory redemption date	Jun. 17, 2034

X

- Definition

Maximum capital required for capital adequacy.

+ References

No definition available.

+ Details

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X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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X

- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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X

- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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X

- Definition

Date the preferred stock has been redeemed, in CCYY-MM-DD format.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

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- Definition

The cash inflow from issuance of preferred stocks by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585

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X

- Definition

Date the subsidiary of the limited liability company (LLC) or limited partnership (LP) was formed, in CCYY-MM-DD format.

+ References

No definition available.

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