XML 17 R1.htm IDEA: XBRL DOCUMENT v3.20.4
Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2020
Jan. 31, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2020    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity Registrant Name COMMUNITY BANK SYSTEM, INC.    
Entity Incorporation, State or Country Code DE    
Entity File Number 001-13695    
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    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Filer Category Large Accelerated Filer    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Small Business false    
Entity Public Float     $ 3,000,356,306
Entity Common Stock, Shares Outstanding   53,675,085  
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000723188    
Document Annual Report true    
Title of 12(b) Security Common Stock, $1.00 par value per share    
Trading Symbol CBU    
Security Exchange Name NYSE    
ICFR Auditor Attestation Flag true