| Document And Entity Information | Jun. 22, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | CORTLAND BANCORP INC |
| Document, Type | 8-K |
| Document, Period End Date | Jun. 22, 2021 |
| Entity, Incorporation, State or Country Code | OH |
| Entity, File Number | 001-38827 |
| Entity, Tax Identification Number | 34-1451118 |
| Entity, Address, Address Line One | 194 West Main Street |
| Entity, Address, City or Town | Cortland |
| Entity, Address, State or Province | OH |
| Entity, Address, Postal Zip Code | 44410 |
| City Area Code | 330 |
| Local Phone Number | 637-8040 |
| 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 | CLDB |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0000774569 |
