ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands		Jun. 12, 2020	Sep. 18, 2019	Jul. 12, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Consideration paid [Abstract]
Cash			$ 500	$ 95,034
Community Bank System, Inc. common stock				0
Total net consideration paid				95,034
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents				90,381
Investment securities				39,770
Loans, net of allowance for credit losses on PCD loans	[1]			479,877
Premises and equipment, net				13,970
Accrued interest and fees receivable				1,130
Other assets				14,109
Deposits				(568,161)
Other liabilities				(2,922)
Other Federal Home Loan Bank borrowings				(2,420)
Subordinated notes payable				(13,831)
Subordinated debt held by unconsolidated subsidiary trusts				(2,062)
Total identifiable assets, net				55,064
Goodwill				39,970	$ 792,715	$ 773,810	$ 733,503
Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles				3,573
Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles				1,650
Steuben [Member]
Consideration paid [Abstract]
Cash		$ 21,613
Community Bank System, Inc. common stock		76,942
Total net consideration paid		98,555
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents		55,973
Investment securities		180,497
Loans, net of allowance for credit losses on PCD loans	[1]	339,164
Premises and equipment, net		8,307
Accrued interest and fees receivable		2,712
Other assets		19,303
Deposits		(516,269)
Other liabilities		(6,436)
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		79,313
Goodwill		19,242
Steuben [Member] | Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		2,928
Steuben [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		$ 1,196
Kinderhook Bank Corp [Member]
Consideration paid [Abstract]
Cash				93,384
Community Bank System, Inc. common stock				0
Total net consideration paid				93,384
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents				90,381
Investment securities				39,770
Loans, net of allowance for credit losses on PCD loans	[1]			479,877
Premises and equipment, net				13,970
Accrued interest and fees receivable				1,130
Other assets				14,109
Deposits				(568,161)
Other liabilities				(2,922)
Other Federal Home Loan Bank borrowings				(2,420)
Subordinated notes payable				(13,831)
Subordinated debt held by unconsolidated subsidiary trusts				(2,062)
Total identifiable assets, net				53,414
Goodwill				39,970
Kinderhook Bank Corp [Member] | Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles				3,573
Kinderhook Bank Corp [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles				0
Other [Member]
Consideration paid [Abstract]
Cash	[2]			1,650
Community Bank System, Inc. common stock	[2]			0
Total net consideration paid	[2]			1,650
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	[2]			0
Investment securities	[2]			0
Loans, net of allowance for credit losses on PCD loans	[1],[2]			0
Premises and equipment, net	[2]			0
Accrued interest and fees receivable	[2]			0
Other assets	[2]			0
Deposits	[2]			0
Other liabilities	[2]			0
Other Federal Home Loan Bank borrowings	[2]			0
Subordinated notes payable	[2]			0
Subordinated debt held by unconsolidated subsidiary trusts	[2]			0
Total identifiable assets, net	[2]			1,650
Goodwill	[2]			0
Other [Member] | Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	[2]			0
Other [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	[2]			$ 1,650

[1] Acquisition-related allowance for credit losses on purchased credit deteriorated (“PCD”) loans applicable beginning in 2020.

[2] Includes amounts related to both acquisitions completed by CISI in 2019.