ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands			12 Months Ended
	Aug. 02, 2021	Jul. 01, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Jun. 12, 2020	Jul. 12, 2019
Consideration :
Cash			$ 29,870		$ 95,034
Community Bank System, Inc. common stock			0		0
Contingent consideration			2,900		0
Total net consideration					95,034
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents			541		90,381
Investment securities			0		39,770
Loans			0		479,877
Premises and equipment, net			760		13,970
Accrued interest and fees receivable			0		1,130
Other assets			579		14,109
Deposits			0		(568,161)
Other liabilities			(1,101)		(2,922)
Other Federal Home Loan Bank borrowings			0		(2,420)
Subordinated notes payable			0		(13,831)
Subordinated debt held by unconsolidated subsidiary trusts			0		(2,062)
Total identifiable assets, net			27,116		55,064
Goodwill			799,109	$ 793,708	773,810
Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			0		3,573
Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			26,337		1,650
Thomas Gregory Associates [Member]
Consideration :
Cash			11,620
Community Bank System, Inc. common stock			0
Contingent consideration	$ 1,500		1,500
Total net consideration			13,120
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents			0
Investment securities			0
Loans			0
Premises and equipment, net			279
Accrued interest and fees receivable			0
Other assets			0
Intangibles	10,900
Deposits			0
Other liabilities			(229)
Other Federal Home Loan Bank borrowings			0
Subordinated notes payable			0
Subordinated debt held by unconsolidated subsidiary trusts			0
Total identifiable assets, net			10,950
Goodwill	$ 2,200		2,170
Thomas Gregory Associates [Member] | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			0
Thomas Gregory Associates [Member] | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			10,900
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member]
Consideration :
Cash			15,350
Community Bank System, Inc. common stock			0
Contingent consideration		$ 1,400	1,400
Total net consideration			16,750
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents			541
Investment securities			0
Loans			0
Premises and equipment, net			282
Accrued interest and fees receivable			0
Other assets			579
Deposits			0
Other liabilities			(698)
Other Federal Home Loan Bank borrowings			0
Subordinated notes payable			0
Subordinated debt held by unconsolidated subsidiary trusts			0
Total identifiable assets, net			14,704
Goodwill		$ 2,000	2,046
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			0
Fringe Benefits Design of Minnesota, Inc ("FBD") [Member] | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			14,000
NuVantage Insurance Corporation [Member]
Consideration :
Cash			2,900
Community Bank System, Inc. common stock			0
Contingent consideration			0
Total net consideration			2,900
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents			0
Investment securities			0
Loans			0
Premises and equipment, net			199
Accrued interest and fees receivable			0
Other assets			0
Deposits			0
Other liabilities			(174)
Other Federal Home Loan Bank borrowings			0
Subordinated notes payable			0
Subordinated debt held by unconsolidated subsidiary trusts			0
Total identifiable assets, net			1,462
Goodwill			1,438
NuVantage Insurance Corporation [Member] | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			0
NuVantage Insurance Corporation [Member] | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			$ 1,437
Kinderhook Bank Corp [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Investment securities							$ 39,800
Loans							479,900
Deposits							(568,200)
Goodwill							$ 40,000
Steuben Trust Corporation [Member]
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 notes payable				0
Subordinated debt held by unconsolidated subsidiary trusts				(2,062)
Total identifiable assets, net				78,574
Goodwill				19,981	$ 39,970	$ 20,000
Steuben Trust Corporation [Member] | Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles				2,928
Steuben Trust Corporation [Member] | Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles				$ 1,196