ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands					3 Months Ended
	Mar. 01, 2017	Feb. 03, 2017	Jan. 04, 2017	Jan. 04, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Consideration paid [Abstract]
Cash					$ 63,517	$ 575
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill					542,986	$ 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
Premises and equipment			0
Accrued interest receivable			0
Other assets			0
Other liabilities			0
Total identifiable assets, net			1,200
Goodwill			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
Premises and equipment		411
Accrued interest receivable		75
Other assets		10,819
Other liabilities		(30,500)
Total identifiable assets, net		72,362
Goodwill		76,194
NRS [Member] | Customer Lists [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles		$ 60,200
Dryfoos [Member]
Consideration paid [Abstract]
Cash	$ 3,307
Community Bank System, Inc. common stock	0
Total net consideration paid	3,307
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	0
Investment securities	0
Premises and equipment	0
Accrued interest receivable	0
Other assets	0
Other liabilities	0
Total identifiable assets, net	1,657
Goodwill	1,650
Dryfoos [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	$ 1,657
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
Premises and equipment				0
Accrued interest receivable				0
Other assets				0
Other liabilities				0
Total identifiable assets, net				288
Goodwill				287
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					74,580
Community Bank System, Inc. common stock					78,483
Total net consideration paid					153,063
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents					11,063
Investment securities					20,294
Premises and equipment					411
Accrued interest receivable					75
Other assets					10,819
Other liabilities					(30,500)
Goodwill					77,844
Total [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles					63,057
Total [Member] | Customer Lists [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Total identifiable assets, net					$ 75,219