ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands						9 Months Ended
		Apr. 02, 2018	May 12, 2017	Feb. 03, 2017	Jan. 01, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jan. 02, 2017
Consideration paid [Abstract]
Cash						$ 1,737	$ 105,402
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill						733,479	$ 731,505	$ 734,430
Other [Member]
Consideration paid [Abstract]
Cash	[1]	$ 1,753
Community Bank System, Inc. common stock	[1]	0
Total net consideration paid	[1]	1,753
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	[1]	16
Investment securities	[1]	0
Loans	[1]	0
Premises and equipment	[1]	10
Accrued interest receivable	[1]	0
Other assets	[1]	105
Deposits	[1]	0
Other liabilities	[1]	(31)
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]	1,443
Goodwill	[1]	310
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]	$ 1,343
BAS [Member]
Consideration paid [Abstract]
Cash					$ 1,200
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
Loans				0
Premises and equipment				411
Accrued interest receivable				72
Other assets				8,088
Deposits				0
Other liabilities				(26,828)
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				73,300
Goodwill				75,256
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			1,488,157
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
Other [Member]
Consideration paid [Abstract]
Cash	[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]					(1,131)
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]					5,444
Goodwill	[2]					3,726
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						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						(39,709)
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						234,850
Goodwill						268,028
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

[1] Includes amounts related to the Penna, Styles Bridges and HR Consultants acquisitions.

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