GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 47,617	$ 48,390
Accumulated Amortization	(31,366)	(30,959)
Net Carrying Amount	16,251	17,431
Finite-lived intangible assets, future amortization expense [Abstract]
Apr - Dec 2013	3,261
2014	3,597
2015	2,804
2016	2,094
2017	1,478
Thereafter	3,017
Total	16,251
Components of goodwill [Abstract]
Goodwill	374,527	374,527
Goodwill, Activity	0
Accumulated Impairment	(4,824)	(4,824)
Accumulated impairment, Activity	0
Goodwill, net	369,703	369,703	345,050
Goodwill, net, Activity	0
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	38,185	38,958
Accumulated Amortization	(24,631)	(24,466)
Net Carrying Amount	13,554	14,492
Other Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	9,432	9,432
Accumulated Amortization	(6,735)	(6,493)
Net Carrying Amount	$ 2,697	$ 2,939

Name:	cbu_ComponentsOfGoodwillAbstract
Namespace Prefix:	cbu_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The accumulated amount of amortization of a major finite-lived intangible asset class. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.