| Document And Entity Information | Jan. 29, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | FNCB Bancorp, Inc. |
| Document, Type | 8-K |
| Document, Period End Date | Jan. 29, 2021 |
| Entity, Incorporation, State or Country Code | PA |
| Entity, File Number | 000-53869 |
| Entity, Tax Identification Number | 23-2900790 |
| Entity, Address, Address Line One | 102 E. Drinker St. |
| Entity, Address, City or Town | Dunmore |
| Entity, Address, State or Province | PA |
| Entity, Address, Postal Zip Code | 18512 |
| City Area Code | 570 |
| Local Phone Number | 346.7667 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock |
| Trading Symbol | FNCB |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001035976 |
