Document and Entity Information |
12 Months Ended |
|---|---|
Dec. 31, 2025 | |
| Document and Entity Information | |
| Document Type | 11-K |
| Entity Registrant Name | Community Financial System, Inc. |
| Entity Central Index Key | 0000723188 |
| Amendment Flag | false |
| Community Financial System, Inc. 401(k) Employee Stock Ownership Plan | |
| Document and Entity Information | |
| Document Type | 11-K |
| Amendment Flag | false |