ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands					9 Months Ended
		May 12, 2017	Feb. 03, 2017	Jan. 04, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Consideration paid [Abstract]
Cash					$ 105,402	$ 575
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill					731,505	$ 465,142	$ 465,142
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 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		$ 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		1,488,680
Premises and equipment		16,608
Accrued interest receivable		4,773
Other assets		51,849
Deposits		(1,448,406)
Other liabilities		(11,774)
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		156,869
Goodwill		188,283
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				$ 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]				4,224
Community Bank System, Inc. common stock	[1]				0
Total net consideration paid	[1]				4,224
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	[1]				0
Investment securities	[1]				0
Loans	[1]				0
Premises and equipment	[1]				27
Accrued interest receivable	[1]				0
Other assets	[1]				272
Deposits	[1]				0
Other liabilities	[1]				(582)
Short-term advances	[1]				0
Securities sold under agreement to repurchase, short-term	[1]				0
Long-term debt	[1]				0
Subordinated debt held by unconsolidated subsidiary trusts	[1]				0
Total identifiable assets, net	[1]				2,574
Goodwill	[1]				1,650
Other [Member] | Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	[1]				0
Other [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	[1]				2,857
Total [Member]
Consideration paid [Abstract]
Cash					157,195
Community Bank System, Inc. common stock					340,737
Total net consideration paid					497,932
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents					51,793
Investment securities					390,942
Loans					1,488,680
Premises and equipment					17,046
Accrued interest receivable					4,845
Other assets					60,209
Deposits					(1,448,406)
Other liabilities					(40,358)
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					231,569
Goodwill					266,363
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					$ 65,914

[1] Includes amounts related to the BAS and Dryfoos acquisitions.