| Cover | Mar. 29, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 29, 2021 |
| Entity File Number | 001-13695 |
| Entity Registrant Name | COMMUNITY BANK SYSTEM, INC. |
| Entity Central Index Key | 0000723188 |
| Entity Tax Identification Number | 16-1213679 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 5790 Widewaters Parkway |
| Entity Address, City or Town | DeWitt |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 13214 |
| City Area Code | 315 |
| Local Phone Number | 445-2282 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $1.00 par value per share |
| Trading Symbol | CBU |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
