ACQUISITIONS - Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands				12 Months Ended
	May 13, 2022	Aug. 02, 2021	Jul. 01, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2022	Jun. 12, 2020
Consideration :
Cash				$ 85,656
Community Bank System, Inc. common stock				0
Contingent consideration				0
Total net consideration				85,656
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents				84,988
Investment securities				11,305
Loans				436,954
Premises and equipment, net				11,317
Accrued interest and fees receivable				884
Other assets				30,007
Other intangibles							$ 2,500
Deposits				522,295
Other liabilities				(3,541)
Other Federal Home Loan Bank borrowings				(17,616)
Subordinated debt held by unconsolidated subsidiary trusts				0
Total identifiable assets, net				42,987
Goodwill				841,841	$ 799,109	$ 793,708
Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Deposits				7,970
Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Other intangibles				3,014
Thomas Gregory Associates Insurance Brokers, Inc.
Consideration :
Contingent consideration		$ 1,500
Fringe Benefits Design of Minnesota, Inc
Consideration :
Contingent consideration			$ 1,400
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill			$ 2,100
Elmira transaction
Consideration :
Cash				82,179
Community Bank System, Inc. common stock				0
Contingent consideration				0
Total net consideration	$ 82,200			82,179
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents				84,988
Investment securities	11,300			11,305
Loans	437,000			436,954
Premises and equipment, net				11,303
Accrued interest and fees receivable				884
Other assets				30,007
Deposits	522,300			522,295
Other liabilities				(3,541)
Other Federal Home Loan Bank borrowings				(17,616)
Subordinated debt held by unconsolidated subsidiary trusts				0
Total identifiable assets, net				39,959
Goodwill	42,200			42,220
Elmira transaction | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Deposits	$ 8,000			7,970
Elmira transaction | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Other intangibles				0
Other
Consideration :
Cash				3,477
Community Bank System, Inc. common stock				0
Contingent consideration				0
Total net consideration				3,477
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents				0
Investment securities				0
Loans				0
Premises and equipment, net				14
Accrued interest and fees receivable				0
Other assets				0
Deposits				0
Other liabilities				0
Other Federal Home Loan Bank borrowings				0
Subordinated debt held by unconsolidated subsidiary trusts				0
Total identifiable assets, net				3,028
Goodwill				449
Other | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Deposits				0
Other | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Other intangibles				$ 3,014
Total
Consideration :
Cash					29,870
Community Bank System, Inc. common stock					0
Contingent consideration					2,900
Total net consideration					32,770
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents					541
Investment securities					0
Loans					0
Premises and equipment, net					760
Accrued interest and fees receivable					0
Other assets					579
Deposits					0
Other liabilities					(1,164)
Other Federal Home Loan Bank borrowings					0
Subordinated debt held by unconsolidated subsidiary trusts					0
Total identifiable assets, net					27,053
Goodwill					5,717
Total | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Deposits					0
Total | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Other intangibles					$ 26,337
Steuben Trust Corporation
Consideration :
Cash						21,613
Community Bank System, Inc. common stock						76,942
Contingent consideration						0
Total net consideration						98,555
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents						55,973
Investment securities						180,497		$ 180,500
Loans						339,017		339,700
Premises and equipment, net						7,764
Accrued interest and fees receivable						2,701
Other assets						17,675
Deposits						516,274		(516,300)
Other liabilities						(4,841)
Other Federal Home Loan Bank borrowings						(6,000)
Subordinated debt held by unconsolidated subsidiary trusts						(2,062)
Total identifiable assets, net						78,574
Goodwill						19,981		$ 20,000
Steuben Trust Corporation | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Deposits						2,928
Steuben Trust Corporation | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Other intangibles						$ 1,196