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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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

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ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands				12 Months Ended
	Jun. 12, 2020	Sep. 18, 2019	Jul. 12, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Consideration paid [Abstract]
Cash		$ 500	$ 95,034	$ (34,360)	$ 4,653	$ 1,737
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			479,877
Premises and equipment			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	793,708	$ 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
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			479,877
Premises and equipment			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			1,650	1,753
Community Bank System, Inc. common stock			0	0
Total net consideration paid			1,650	1,753
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents			0	16
Investment securities			0	0
Loans			0	0
Premises and equipment			0	10
Accrued interest and fees receivable			0	0
Other assets			0	105
Deposits			0	0
Other liabilities			0	(31)
Other Federal Home Loan Bank borrowings			0	0
Subordinated notes payable			0	0
Subordinated debt held by unconsolidated subsidiary trusts			0	0
Total identifiable assets, net			1,650	1,443
Goodwill			0	310
Other [Member] \| Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			0	0
Other [Member] \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			$ 1,650	1,343
Steuben Trust Corporation [Member]
Consideration paid [Abstract]
Cash	$ 98,600			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	180,500			180,497
Loans	339,700			339,017
Premises and equipment				7,764
Accrued interest and fees receivable				2,701
Other assets				17,675
Deposits	516,300			(516,274)
Other liabilities				(5,095)
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				78,320
Goodwill	$ 20,200			20,235
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