ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands				12 Months Ended
	Dec. 04, 2015	Jan. 02, 2014	Dec. 13, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Consideration paid (received) [Abstract]
Total net consideration paid (received)				$ (25,505)	$ 924	$ (291,980)
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill				$ 463,252	$ 375,174	$ 374,991
EBS-RMSCO, Inc. [Member]
Consideration paid (received) [Abstract]
Cash		$ 924
Community Bank System, Inc. common stock		0
Total net consideration paid (received)		924
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		163
Core deposit intangibles		0
Other intangibles		578
Deposits		0
Other liabilities		0
Total identifiable assets (liabilities), net		741
Goodwill		$ 183
Bank of America Agreement [Member]
Consideration paid (received) [Abstract]
Cash			$ (291,980)
Community Bank System, Inc. common stock			0
Total net consideration paid (received)			(291,980)
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents			0
Investment securities			0
Loans			1,106
Premises and equipment			2,549
Accrued interest receivable			5
Other assets			(18)
Core deposit intangibles			2,537
Other intangibles			9
Deposits			(303,456)
Other liabilities			0
Total identifiable assets (liabilities), net			(297,268)
Goodwill			$ 5,288
Oneida Financial Corp [Member]
Consideration paid (received) [Abstract]
Cash	$ 56,266
Community Bank System, Inc. common stock	102,202
Total net consideration paid (received)	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	27,452
Core deposit intangibles	2,570
Other intangibles	9,994
Deposits	(699,241)
Other liabilities	(653)
Total identifiable assets (liabilities), net	70,390
Goodwill	$ 88,078