| Document and Entity Information | Oct. 20, 2021 |
| Cover [Abstract] | |
| Security Exchange Name | NASDAQ |
| Amendment Flag | false |
| Entity Central Index Key | 0000774569 |
| Document Type | 8-K |
| Document Period End Date | Oct. 20, 2021 |
| Entity Registrant Name | CORTLAND BANCORP INC |
| Entity Incorporation State 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 | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, No Par Value |
| Trading Symbol | CLDB |
| Entity Emerging Growth Company | false |
