Amount of deposits acquired in an acquisition.

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Amount of loans acquired in an acquisition.

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Amount of Federal Home Loan Bank borrowings due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of subordinated notes payable due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of subordinated debt held by unconsolidated subsidiary trusts due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

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Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Amount due from customers or clients for goods or services, including trade receivables, that have been delivered or sold in the normal course of business, and amounts due from others, including related parties expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.

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The amount of identifiable intangible assets recognized as of the acquisition date.

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Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

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Amount of other liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of other assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.

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The amount of property, plant, and equipment recognized as of the acquisition date.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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ACQUISITIONS - Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Jun. 12, 2020	Jul. 12, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Consideration paid [Abstract]
Cash		$ 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		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	$ 793,749	$ 773,810	$ 773,610
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
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	339,017
Premises and equipment, net	7,764
Accrued interest and fees receivable	2,712
Other assets	17,633
Deposits	(516,274)
Other liabilities	(5,105)
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	78,279
Goodwill	20,276
Steuben \| Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	2,928
Steuben \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	$ 1,196
Kinderhook Bank Corp
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		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 \| Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		3,573
Kinderhook Bank Corp \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		0
Other
Consideration paid [Abstract]
Cash		1,650
Community Bank System, Inc. common stock		0
Total net consideration paid		1,650
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents		0
Investment securities		0
Loans, net of allowance for credit losses on PCD loans		0
Premises and equipment, net		0
Accrued interest and fees receivable		0
Other assets		0
Deposits		0
Other liabilities		0
Other Federal Home Loan Bank borrowings		0
Subordinated notes payable		0
Subordinated debt held by unconsolidated subsidiary trusts		0
Total identifiable assets, net		1,650
Goodwill		0
Other \| Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		0
Other \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		$ 1,650