v3.26.1
Document and Entity Information - shares
 6 Months Ended
Jun. 30, 2026
Jul. 31, 2026
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
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 333 Butternut Drive  
Entity Address, City or Town Syracuse  
Entity Address, State or Province NY  
Entity Address, Postal Zip Code 13214-2141  
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 Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   52,615,905
Entity Central Index Key 0000723188  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q2  
Amendment Flag false