Note 12 - Commitments, Contingencies and Concentrations (Details Textual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Operating Leases, Rent Expense, Net, Total	$ 418	$ 492
Reserves for Unfunded Commitments	703	255
Payments Serviced for Federal Home Loan Bank	21,500
Maximum Obligation for Guarantees to Federal Home Loan Bank if Aggregate Foreclosure Losses on Pool of Loans Exceeds Specified Amount	902
Maximum Amount of Aggregate Foreclosure Losses on Entire Pool of Loans	64
Cash and Due from Banks, Total	22,861	26,673
Loans and Leases Receivable, Gross, Total	826,356	835,207
Commercial Real Estate, Construction Land Acquisition, and Development Loans Portfolio [Member] \| Financing Receivable [Member] \| Credit Concentration Risk [Member]
Loans and Leases Receivable, Gross, Total	$ 325,600
Loans and Leases Receivable as Percentage of Aggregate Gross Loans and Leases Receivable	39.40%
Commercial Real Estate, Construction Land Acquisition, and Development Loans Portfolio [Member] \| Financing Receivable [Member] \| Geographic Concentration Risk [Member]
Loans and Leases Receivable, Gross, Total	$ 24,300
Loans and Leases Receivable as Percentage of Aggregate Gross Loans and Leases Receivable	2.90%
Compass Bank [Member]
Cash and Due from Banks, Total	$ 1,100	$ 2,700
Other Assets [Member]
Operating Lease, Right-of-Use Asset	3,100
Other Liabilities [Member]
Operating Lease, Liability, Total	$ 3,471

Represents the amount of loans as a percentage of aggregate gross loan portfolio.

No definition available.

Name:	fncb_LoansAndLeasesReceivableAsPercentageOfAggregateGrossLoansAndLeasesReceivable
Namespace Prefix:	fncb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum amount of total foreclosure losses on the entire pool of loans.

+ References

No definition available.

+ Details

Name:	fncb_MaximumAmountOfAggregateForeclosureLossesOnEntirePoolOfLoans
Namespace Prefix:	fncb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the maximum obligation for credit enhancement guarantees by the entity to the Federal Home Loan Bank, if total foreclosure losses on the entire pool of loans exceed specified amount.

+ References

No definition available.

+ Details

Name:	fncb_MaximumObligationForGuaranteesToFederalHomeLoanBankIfAggregateForeClosureLossesOnPoolOfLoansExceedsSpecifiedAmount
Namespace Prefix:	fncb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the cash outflow due to servicing of payments for Federal Home Loan Bank guarantee.

+ References

No definition available.

+ Details

Name:	fncb_PaymentsServicedForFederalHomeLoanBank
Namespace Prefix:	fncb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reserves For Unfunded Commitments.

+ References

No definition available.

+ Details

Name:	fncb_ReservesForUnfundedCommitments
Namespace Prefix:	fncb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For banks and other depository institutions: Includes cash on hand (currency and coin), cash items in process of collection, noninterest bearing deposits due from other financial institutions (including corporate credit unions), and noninterest bearing balances with the Federal Reserve Banks, Federal Home Loan Banks and central banks.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(1))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

+ Details

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

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fncb_CommercialRealEstateConstructionLandAcquisitionAndDevelopmentLoansPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=fncb_FinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CreditConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_GeographicConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=fncb_CompassBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: