Note 11 - Related Party Transactions (Details Textual) - USD ($) $ in Thousands		12 Months Ended
Feb. 08, 2019	Dec. 31, 2019	Dec. 31, 2018
Related Party Transactions Interest Accrued and Unpaid			$ 12
Directors, Executive Officers and Their Related Parties [Member]
Related Party Deposit Liabilities		$ 84,100	115,500
Related Party Transactions, Interest Paid on Deposits		484	348
Related Party Transaction, Expenses from Transactions with Related Party		2,400	2,500
Related Party Transaction, Subordinated Debt, Accelerated Partial Principal Repayment	$ 5,000
Related Party Transaction, Interest Paid	$ 3,100
Due to Related Parties, Total		0
Related Party Transactions, Subordinated Debt			3,100
Interest Expense, Related Party		24	$ 141
Directors, Executive Officers and Their Related Parties [Member] \| Nonperforming Financial Instruments [Member]
Due from Related Parties, Total		$ 0

Cash payments for interest on related party transactions.

No definition available.

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

- Definition

Represents the amount accelerated partial principal repayment for subordinated debt with related party.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of interest accrued and unpaid on loans given to the related party.

+ References

No definition available.

+ Details

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

- Definition

Represents the aggregate amount of interest paid on deposits to the related party.

+ References

No definition available.

+ Details

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

- Definition

Represents the carrying value as of the balance sheet date of subordinated debt held by related parties.

+ References

No definition available.

+ Details

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

- Definition

For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

+ 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 210.9-03.10(3))
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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Definition

Carrying amount as of the balance sheet date of obligations due all related parties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of interest expense incurred on a debt or other obligation to related party.

+ References

No definition available.

+ Details

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

- Definition

Amount of deposits held by the entity for a related party (entity, shareholder, employee).

+ References

+ Details

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

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: