| Cover | Jun. 30, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jun. 30, 2020 |
| Current Fiscal Year End Date | --12-31 |
| Entity File Number | 001-09279 |
| Entity Registrant Name | ONE LIBERTY PROPERTIES, INC. |
| Entity Central Index Key | 0000712770 |
| Entity Tax Identification Number | 13-3147497 |
| Entity Incorporation, State or Country Code | MD |
| Entity Address, Address Line One | 60 Cutter Mill Road |
| Entity Address, Address Line Two | Suite 303 |
| Entity Address, City or Town | Great Neck |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 11021 |
| City Area Code | (516) |
| Local Phone Number | 466-3100 |
| 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 | OLP |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
