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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2024
Jan. 31, 2025
Jun. 28, 2024
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2024    
Document Transition Report false    
Securities Act File Number 001-13695    
Entity Registrant Name COMMUNITY FINANCIAL SYSTEM, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 16-1213679    
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    
Title of 12(b) Security Common Stock, $1.00 par value per share    
Trading Symbol CBU    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 2,455,654,871
Entity Common Stock, Shares Outstanding   52,712,324  
Entity Central Index Key 0000723188    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2024    
Document Fiscal Period Focus FY    
Amendment Flag false    
Auditor Name PricewaterhouseCoopers LLP    
Auditor Firm ID 238    
Auditor Location New York