Allowance for Loan Losses - Allowance by impairment Methodology and Loans by impairment methodology (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Allowance for loan losses:
Beginning Balance	$ 13,075	$ 12,937	$ 12,937
Charge-offs	(3,345)	(1,784)	(3,793)
Recoveries	498	196	376
Provision for (recovery of) loan losses	2,250	2,650	3,555
Ending Balance	12,478	13,999	13,075
Ending Balance:
Allowance for loan losses, Individually evaluated for impairment	1,122	3,205	1,838
Allowance for loan losses, Collectively evaluated for impairment	11,356	10,794	11,237
Loans
Total	377,513	384,135	383,594
Loans, Individually evaluated for impairment	10,844	21,931	16,980
Loans, Collectively evaluated for impairment	366,669	362,204	366,614
Construction and Land Development [Member]
Allowance for loan losses:
Beginning Balance	2,481	2,843	2,843
Charge-offs	(2,308)	(369)	(431)
Recoveries		1	1
Provision for (recovery of) loan losses	3,097	700	68
Ending Balance	3,270	3,175	2,481
Ending Balance:
Allowance for loan losses, Individually evaluated for impairment	747	1,145	567
Allowance for loan losses, Collectively evaluated for impairment	2,523	2,030	1,914
Loans
Total	44,305	47,843	43,524
Loans, Individually evaluated for impairment	3,595	5,256	2,516
Loans, Collectively evaluated for impairment	40,710	42,587	41,008
Secured by 1-4 Family Residential [Member]
Allowance for loan losses:
Beginning Balance	3,712	3,766	3,766
Charge-offs	(172)	(562)
Recoveries	6	2	68
Provision for (recovery of) loan losses	179	993	639
Ending Balance	3,725	4,199	3,712
Ending Balance:
Allowance for loan losses, Individually evaluated for impairment	195	972	306
Allowance for loan losses, Collectively evaluated for impairment	3,530	3,227	3,406
Loans
Total	145,628	128,229	134,964
Loans, Individually evaluated for impairment	1,940	5,460	3,776
Loans, Collectively evaluated for impairment	143,688	122,769	131,188
Other Real Estate [Member]
Allowance for loan losses:
Beginning Balance	6,163	5,192	5,192
Charge-offs	(798)	(674)	(2,154)
Recoveries	206	52	64
Provision for (recovery of) loan losses	(561)	1,285	3,061
Ending Balance	5,010	5,855	6,163
Ending Balance:
Allowance for loan losses, Individually evaluated for impairment	161	1,033	930
Allowance for loan losses, Collectively evaluated for impairment	4,849	4,822	5,233
Loans
Total	159,834	174,212	176,573
Loans, Individually evaluated for impairment	5,219	10,813	10,528
Loans, Collectively evaluated for impairment	154,615	163,399	166,045
Commercial and Industrial [Member]
Allowance for loan losses:
Beginning Balance	608	963	963
Charge-offs		(74)	(261)
Recoveries	176	16	35
Provision for (recovery of) loan losses	(399)	(264)	(129)
Ending Balance	385	641	608
Ending Balance:
Allowance for loan losses, Individually evaluated for impairment	19	55	35
Allowance for loan losses, Collectively evaluated for impairment	366	586	573
Loans
Total	19,701	24,937	20,718
Loans, Individually evaluated for impairment	90	402	160
Loans, Collectively evaluated for impairment	19,611	24,535	20,558
Consumer and Other Loans [Member]
Allowance for loan losses:
Beginning Balance	111	173	173
Charge-offs	(67)	(105)	(186)
Recoveries	110	125	208
Provision for (recovery of) loan losses	(66)	(64)	(84)
Ending Balance	88	129	111
Ending Balance:
Allowance for loan losses, Individually evaluated for impairment
Allowance for loan losses, Collectively evaluated for impairment	88	129	111
Loans
Total	8,045	8,914	7,815
Loans, Individually evaluated for impairment
Loans, Collectively evaluated for impairment	$ 8,045	$ 8,914	$ 7,815

Name:	fxnc_FinancingReceivableLoansAndLeasesReceivableAbstract
Namespace Prefix:	fxnc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5212-111524

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

+ Details

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

- Definition

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

+ References

No definition available.

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953423-111524

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953423-111524

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ 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 210.9-03.7(d))

-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 7

-Subparagraph d

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(1)(ii)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

+ Details

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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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