ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands								12 Months Ended
	Sep. 18, 2019	Jul. 12, 2019		Apr. 02, 2018	[2]	May 12, 2017	Feb. 03, 2017	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Consideration paid [Abstract]
Cash	$ 500	$ 95,034						$ 4,653		$ 1,737	$ 107,414
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		(3,259)
Short-term advances		0
Securities sold under agreement to repurchase, short-term		0
Other Federal Home Loan Bank borrowings		(2,420)
Subordinated notes payable		(13,831)						0
Subordinated debt held by unconsolidated subsidiary trusts		(2,062)
Total identifiable assets, net		54,727
Goodwill		40,307						773,810		$ 733,503	$ 734,430
Core Deposits [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		(3,259)
Short-term advances		0
Securities sold under agreement to repurchase, short-term		0
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		53,077
Goodwill		40,307
Kinderhook Bank Corp [Member] | Core Deposits [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]	$ 1,753				6,775	[3]
Community Bank System, Inc. common stock		0	[1]	0				2,395	[3]
Total net consideration paid		1,650	[1]	1,753				9,170	[3]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents		0	[1]	16				339	[3]
Investment securities		0	[1]	0				0	[3]
Loans		0	[1]	0				0	[3]
Premises and equipment		0	[1]	10				27	[3]
Accrued interest and fees receivable		0	[1]	0				0	[3]
Other assets		0	[1]	105				583	[3]
Deposits		0	[1]	0				0	[3]
Other liabilities		0	[1]	(31)				(1,155)	[3]
Short-term advances		0	[1]	0				0	[3]
Securities sold under agreement to repurchase, short-term		0	[1]	0				0	[3]
Other Federal Home Loan Bank borrowings		0	[1]	0				0	[3]
Subordinated notes payable		0	[1]	0				0	[3]
Subordinated debt held by unconsolidated subsidiary trusts		0	[1]	0				0	[3]
Total identifiable assets, net		1,650	[1]	1,443				5,420	[3]
Goodwill		0	[1]	310				3,750	[3]
Other [Member] | Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		0	[1]	0				0	[3]
Other [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		$ 1,650	[1]	$ 1,343				5,626	[3]
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 and fees receivable							72
Other assets							8,088
Deposits							0
Other liabilities							(26,828)
Short-term advances							0
Securities sold under agreement to repurchase, short-term							0
Other Federal Home Loan Bank borrowings							0
Subordinated notes payable							0
Subordinated debt held by unconsolidated subsidiary trusts							0
Total identifiable assets, net							73,300
Goodwill							75,256
NRS [Member] | Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles							0
NRS [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles							$ 60,200
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						370,648
Loans						1,488,157
Premises and equipment						16,608
Accrued interest and fees receivable						4,773
Other assets						51,585
Deposits						(1,448,406)
Other liabilities						(11,750)
Short-term advances						(80,000)
Securities sold under agreement to repurchase, short-term						(278,076)
Other Federal Home Loan Bank borrowings						(3,615)
Subordinated notes payable						0
Subordinated debt held by unconsolidated subsidiary trusts						(20,619)
Total identifiable assets, net						156,106
Goodwill						189,046
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
Total [Member]
Consideration paid [Abstract]
Cash								159,746
Community Bank System, Inc. common stock								343,132
Total net consideration paid								502,878
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents								52,132
Investment securities								390,942
Loans								1,488,157
Premises and equipment								17,046
Accrued interest and fees receivable								4,845
Other assets								60,256
Deposits								(1,448,406)
Other liabilities								(39,733)
Short-term advances								(80,000)
Securities sold under agreement to repurchase, short-term								(278,076)
Other Federal Home Loan Bank borrowings								(3,615)
Subordinated debt held by unconsolidated subsidiary trusts								(20,619)
Total identifiable assets, net								234,826
Goodwill								268,052
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								$ 68,683

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

[2] Includes amounts related to the Penna, Styles Bridges and HR Consultants acquisitions.

[3] Includes amounts related to the BAS, Dryfoos, NECM and GBR acquisitions.