ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands			9 Months Ended
	Jan. 04, 2016	Dec. 04, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Consideration paid [Abstract]
Total net consideration paid			$ 575	$ 0
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill			$ 465,142	$ 375,174	$ 463,252
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
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
Oneida Financial Corp [Member]
Consideration paid [Abstract]
Cash		$ 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)
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