Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Oct. 31, 2023
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2023
Entity File Number	001-13695
Entity Registrant Name	COMMUNITY BANK 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 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		53,385,565
Entity Central Index Key	0000723188
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Amendment Flag	false