ACQUISITIONS - Estimated Fair Value of Assets Acquired and Liabilities assumed (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 12, 2020	Dec. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill		$ 793,708	$ 793,456	$ 773,473
Steuben Trust Corporation [Member]
Consideration paid [Abstract]
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 [Abstract]
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		(4,843)
Other Federal Home Loan Bank borrowings		(6,000)
Subordinated debt held by unconsolidated subsidiary trusts		(2,062)
Total identifiable assets, net		78,572
Goodwill	$ 20,000	19,983
Steuben Trust Corporation [Member] | Core deposit intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		2,928
Steuben Trust Corporation [Member] | Other intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		$ 1,196