ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands						12 Months Ended
	Apr. 02, 2018	[1]	May 12, 2017	Feb. 03, 2017	Jan. 04, 2016	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Consideration paid [Abstract]
Cash						$ 1,737		$ 107,414	$ 575
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Goodwill						733,503		$ 734,430	$ 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				(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
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	$ 1,753					6,775	[2]
Community Bank System, Inc. common stock	0					2,395	[2]
Total net consideration paid	1,753					9,170	[2]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	16					339	[2]
Investment securities	0					0	[2]
Loans	0					0	[2]
Premises and equipment	10					27	[2]
Accrued interest receivable	0					0	[2]
Other assets	105					583	[2]
Deposits	0					0	[2]
Other liabilities	(31)					(1,155)	[2]
Short-term advances	0					0	[2]
Securities sold under agreement to repurchase, short-term	0					0	[2]
Long-term debt	0					0	[2]
Subordinated debt held by unconsolidated subsidiary trusts	0					0	[2]
Total identifiable assets, net	1,443					5,420	[2]
Goodwill	310					3,750	[2]
Other [Member] | Core Deposits [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	0					0	[2]
Other [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	$ 1,343					5,626	[2]
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						(39,733)
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						234,826
Goodwill						268,052
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.