ACQUISITIONS - Estimated Fair Value of Assets Acquired and Liabilities assumed (Details) - USD ($) $ in Thousands					9 Months Ended
	Aug. 02, 2021	Jul. 01, 2021	Jun. 01, 2021	Jun. 12, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Consideration :
Cash			$ 29,870		$ 29,329	$ (34,360)
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, net of allowance for credit losses on PCD loans			0
Premises and equipment, net			760
Accrued interest and fees receivable			0
Other assets			564
Deposits			0
Other liabilities			(1,101)
Other Federal Home Loan Bank borrowings			0
Subordinated debt held by unconsolidated subsidiary trusts			0
Total identifiable assets, net			27,101
Goodwill			5,669		$ 799,127	$ 793,749	$ 793,708
Core deposit intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			0
Other intangibles
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles			26,337
Thomas Gregory Associates [Member]
Consideration :
Cash	$ 11,620
Community Bank System, Inc. common stock	0
Contingent consideration	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, net of allowance for credit losses on PCD loans	0
Premises and equipment, net	279
Accrued interest and fees receivable	0
Other assets	0
Deposits	0
Other liabilities	(229)
Other Federal Home Loan Bank borrowings	0
Subordinated debt held by unconsolidated subsidiary trusts	0
Total identifiable assets, net	10,950
Goodwill	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
Total net consideration		16,750
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents		541
Investment securities		0
Loans, net of allowance for credit losses on PCD loans		0
Premises and equipment, net		282
Accrued interest and fees receivable		0
Other assets		564
Deposits		0
Other liabilities		(698)
Other Federal Home Loan Bank borrowings		0
Subordinated debt held by unconsolidated subsidiary trusts		0
Total identifiable assets, net		14,689
Goodwill		2,061
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, net of allowance for credit losses on PCD 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 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
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
Loans, net of allowance for credit losses on PCD loans				339,017
Premises and equipment, net				7,764
Accrued interest and fees receivable				2,701
Other assets				17,675
Deposits				(516,274)
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
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