Note 15 - Regulatory Matters/Subsequent Events - Risk-based Capital and Related Ratios (Details) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Total capital (to risk-weighted assets), Amount	$ 169,984	$ 161,957
Total capital (to risk-weighted assets), Ratio	0.1311	0.1464
Total capital (to risk-weighted assets), Minimum required for capital adequacy purposes	0.0800	0.0800
Total capital (to risk-weighted assets), Minimum required for capital adequacy purposes with conservation buffer	0.1050	0.1050
Total capital (to risk-weighted assets), To be well capitalized under prompt corrective action	0.1000	0.1000
Tier I capital (to risk-weighted assets), Amount	$ 154,842	$ 148,958
Tier I capital (to risk-weighted assets), Ratio	0.1194	0.1346
Tier I capital (to risk-weighted assets), Minimum required for capital adequacy purposes	0.0600	0.0600
Tier I capital (to risk-weighted assets), Minimum required for capital adequacy purposes with conservation buffer	0.0850	0.0850
Tier I capital (to risk-weighted assets), To be well capitalized under prompt corrective action	0.0800	0.0800
Tier I common equity (to risk-weighted assets), Amount	$ 154,842	$ 148,958
Tier I common equity (to risk-weighted assets), Ratio	0.1194	0.1346
Tier I common equity (to risk-weighted assets), Minimum required for capital adequacy purposes	0.0450	0.0450
Tier I common equity (to risk-weighted assets), Minimum required for capital adequacy purposes with conservation buffer	0.0700	0.0700
Tier I common equity (to risk-weighted assets), To be well capitalized under prompt corrective action	0.0650	0.0650
Tier I capital (to average assets), Amount	$ 154,842	$ 148,958
Tier I capital (to average assets), Ratio	0.0877	0.0892
Tier I capital (to average assets), Minimum required for capital adequacy purposes	0.0400	0.0400
Tier I capital (to average assets), Minimum required for capital adequacy purposes with conservation buffer	0.0400	0.0400
Tier I capital (to average assets), To be well capitalized under prompt corrective action	0.0500	0.0500
Total risk-weighted assets, Amount	$ 1,296,618	$ 1,106,636
Total average assets, Amount	$ 1,765,251	$ 1,669,932

Amount of average assets as calculated in accordance with U.S. Federal regulatory capital standards.

No definition available.

Name:	fncb_AverageAssets
Namespace Prefix:	fncb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The minimum total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action, plus the capital conservation buffer (phased in at 0.625% annually).

+ References

No definition available.

+ Details

Name:	fncb_CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsWithCapitalConservationBuffer
Namespace Prefix:	fncb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The minimum Common Equity Tier One Capital Ratio (Common Equity Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action, plus the fully phased-in capital conservation buffer (phased in at 0.625% annually).

+ References

No definition available.

+ Details

Name:	fncb_CommonEquityTierOneRiskBasedcapitalRequiredForCapitalAdequacyToRiskWeightedAssetsWithCapitalConservationBuffer
Namespace Prefix:	fncb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations, plus the capital conservation buffer (phased in at 0.625% annually).

+ References

No definition available.

+ Details

Name:	fncb_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsWithCapitalConservationBuffer
Namespace Prefix:	fncb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action, plus the fully phased in capital conservation buffer (phased in at 0.625% annually).

+ References

No definition available.

+ Details

Name:	fncb_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsWithCapitalConservationBuffer
Namespace Prefix:	fncb_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum Common Equity Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI https://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

+ Details

Name:	us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioCapitalAdequacyMinimum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum Common Equity Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

+ Details

Name:	us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of total capital as defined by regulatory framework.

+ References

+ Details

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

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of Common Equity Tier 1 risk-based capital as defined by regulatory framework.

+ References

+ Details

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

- Definition

Ratio of Common Equity Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CommonEquityTierOneCapitalRatio
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of risk-weighted assets as defined by regulatory framework.

+ References

+ Details

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

- Definition

Amount of Tier 1 leverage capital as defined by regulatory framework.

+ References

+ Details

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

- Definition

Ratio of minimum Tier 1 leverage capital to average assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum Tier 1 leverage capital to average assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 capital to average assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of Tier 1 risk-based capital as defined by regulatory framework.

+ References

+ Details

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

- Definition

Ratio of minimum Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant