Amount of deposits acquired in an acquisition.

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Amount of loans acquired in an acquisition.

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Amount of securities sold under agreement to repurchase due within one year or within the normal operating cycle, assumed at the acquisition date.

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Amount of short-term advances due within one year or within the normal operating cycle, 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 long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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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						6 Months Ended
	May 12, 2017	Mar. 01, 2017	Feb. 03, 2017	Jan. 04, 2017	Jan. 04, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Consideration paid [Abstract]
Cash						$ 105,402	$ 575
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill						733,604	$ 465,142	$ 465,142
BAS [Member]
Consideration paid [Abstract]
Cash				$ 1,200
Community Bank System, Inc. common stock				0
Total net consideration paid				1,200
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents				0
Investment securities				0
Loans				0
Premises and equipment				0
Accrued interest receivable				0
Other assets				0
Deposits				0
Other liabilities				0
Short-term advances				0
Securities sold under agreement to repurchase, short-term				0
Long-term debt				0
Subordinated debt held by unconsolidated subsidiary trusts				0
Total identifiable assets, net				1,200
Goodwill				0
BAS [Member] \| Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles				0
BAS [Member] \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles				$ 1,200
NRS [Member]
Consideration paid [Abstract]
Cash			$ 70,073
Community Bank System, Inc. common stock			78,483
Total net consideration paid			148,556
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents			11,063
Investment securities			20,294
Loans			0
Premises and equipment			411
Accrued interest receivable			72
Other assets			8,088
Deposits			0
Other liabilities			(28,106)
Short-term advances			0
Securities sold under agreement to repurchase, short-term			0
Long-term debt			0
Subordinated debt held by unconsolidated subsidiary trusts			0
Total identifiable assets, net			72,022
Goodwill			76,534
NRS [Member] \| Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			0
NRS [Member] \| Customer Lists [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			$ 60,200
Dryfoos [Member]
Consideration paid [Abstract]
Cash		$ 3,024
Community Bank System, Inc. common stock		0
Total net consideration paid		3,024
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents		0
Investment securities		0
Loans		0
Premises and equipment		27
Accrued interest receivable		0
Other assets		272
Deposits		0
Other liabilities		(582)
Short-term advances		0
Securities sold under agreement to repurchase, short-term		0
Long-term debt		0
Subordinated debt held by unconsolidated subsidiary trusts		0
Total identifiable assets, net		1,374
Goodwill		1,650
Dryfoos [Member] \| Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		0
Dryfoos [Member] \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		$ 1,657
Merchants Bancshares, Inc. [Member]
Consideration paid [Abstract]
Cash	$ 82,898
Community Bank System, Inc. common stock	262,254
Total net consideration paid	345,152
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	40,730
Investment securities	370,480
Loans	1,489,273
Premises and equipment	12,973
Accrued interest receivable	4,773
Other assets	46,363
Deposits	(1,448,408)
Other liabilities	(5,071)
Short-term advances	(80,000)
Securities sold under agreement to repurchase, short-term	(278,076)
Long-term debt	(3,615)
Subordinated debt held by unconsolidated subsidiary trusts	(20,619)
Total identifiable assets, net	154,874
Goodwill	190,278
Merchants Bancshares, Inc. [Member] \| Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	23,214
Merchants Bancshares, Inc. [Member] \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	$ 2,857
WJL Agencies Inc [Member]
Consideration paid [Abstract]
Cash					$ 575
Community Bank System, Inc. common stock					0
Total net consideration paid					575
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents					0
Investment securities					0
Loans					0
Premises and equipment					0
Accrued interest receivable					0
Other assets					0
Deposits					0
Other liabilities					0
Short-term advances					0
Securities sold under agreement to repurchase, short-term					0
Long-term debt					0
Subordinated debt held by unconsolidated subsidiary trusts					0
Total identifiable assets, net					288
Goodwill					287
WJL Agencies Inc [Member] \| Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles					0
WJL Agencies Inc [Member] \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles					$ 288
Total [Member]
Consideration paid [Abstract]
Cash						157,195
Community Bank System, Inc. common stock						340,737
Total net consideration paid						497,932
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents						51,793
Investment securities						390,774
Loans						1,489,273
Premises and equipment						13,411
Accrued interest receivable						4,845
Other assets						54,723
Deposits						(1,448,408)
Other liabilities						(33,759)
Short-term advances						(80,000)
Securities sold under agreement to repurchase, short-term						(278,076)
Long-term debt						(3,615)
Subordinated debt held by unconsolidated subsidiary trusts						(20,619)
Total identifiable assets, net						229,470
Goodwill						268,462
Total [Member] \| Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles						23,214
Total [Member] \| Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles						$ 65,914