ACQUISITIONS - Estimated Fair Value of Assets Acquired and Liabilities assumed (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
	Jun. 12, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Consideration paid :
Cash		$ 2,900	$ (34,360)
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill		794,892	$ 792,715	$ 793,708
Steuben Trust Corporation [Member]
Consideration paid :
Cash	$ 21,600			21,613
Community Bank System, Inc. common stock				76,942
Total net consideration paid	98,600			98,555
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents				55,973
Investment securities	180,500			180,497
Loans, net of allowance for credit losses on PCD loans	339,700			339,017
Premises and equipment, net				7,764
Accrued interest and fees receivable				2,701
Other assets				17,675
Deposits	(516,300)			(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	$ 20,000			19,981
Steuben Trust Corporation [Member] | Core deposit intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles				2,928
Steuben Trust Corporation [Member] | Other intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles				$ 1,196
NuVantage Insurance Corporation [Member]
Consideration paid :
Cash		2,900
Community Bank System, Inc. common stock		0
Total net consideration paid		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 [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles		0
NuVantage Insurance Corporation [Member] | Other intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangibles		$ 1,437