| Document And Entity Information | Oct. 29, 2020 |
| Document And Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 29, 2020 |
| Entity Registrant Name | The First of Long Island Corporation |
| Entity Incorporation, State or Country Code | NY |
| Entity File Number | 001-32964 |
| Entity Tax Identification Number | 11-2672906 |
| Entity Address, Address Line One | 10 Glen Head Road |
| Entity Address, City or Town | Glen Head |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 11545 |
| City Area Code | 516 |
| Local Phone Number | 671-4900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, $0.10 par value per share |
| Trading Symbol | FLIC |
| Security Exchange Name | NASDAQ |
| Entity Central Index Key | 0000740663 |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
