ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands						12 Months Ended
		May 12, 2017	Feb. 03, 2017	Jan. 04, 2016	Dec. 04, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consideration paid [Abstract]
Cash						$ 107,414	$ 575	$ (25,505)
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill						734,430	$ 465,142	$ 463,252
NRS [Member]
Consideration paid [Abstract]
Cash	[1]		$ 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			(28,002)
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			72,126
Goodwill			76,430
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	[1]	$ 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 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)
Long-term debt		(3,615)
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
WJL Agencies Inc [Member]
Consideration paid [Abstract]
Cash	[1]			$ 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
Other [Member]
Consideration paid [Abstract]
Cash	[1],[2]					6,775
Community Bank System, Inc. common stock	[2]					2,395
Total net consideration paid	[2]					9,170
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	[2]					339
Investment securities	[2]					0
Loans	[2]					0
Premises and equipment	[2]					27
Accrued interest receivable	[2]					0
Other assets	[2]					583
Deposits	[2]					0
Other liabilities	[2]					(1,217)
Short-term advances	[2]					0
Securities sold under agreement to repurchase, short-term	[2]					0
Long-term debt	[2]					0
Subordinated debt held by unconsolidated subsidiary trusts	[2]					0
Total identifiable assets, net	[2]					5,358
Goodwill	[2]					3,812
Other [Member] | Core Deposits [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]					5,626
Total [Member]
Consideration paid [Abstract]
Cash	[1]					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 receivable						4,845
Other assets						60,256
Deposits						(1,448,406)
Other liabilities						(40,969)
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						233,590
Goodwill						269,288
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
Oneida Financial Corp [Member]
Consideration paid [Abstract]
Cash	[1]				$ 56,266
Community Bank System, Inc. common stock					102,202
Total net consideration paid					158,468
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents					81,772
Investment securities					225,729
Loans					399,422
Premises and equipment					22,212
Accrued interest receivable					1,133
Other assets					26,529
Deposits					(699,241)
Other liabilities					(1,333)
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					68,787
Goodwill					89,681
Oneida Financial Corp [Member] | Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles					2,570
Oneida Financial Corp [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles					$ 9,994

[1] Includes NECM $0.2 million contingent cash payment consideration.

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