GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 41,868	$ 41,866
Accumulated Amortization	(28,483)	(26,352)
Net Carrying Amount	13,385	15,514
Finite-lived intangible assets, future amortization expense [Abstract]
Jul - Dec 2012	1,982
2013	3,305
2014	2,579
2015	1,905
2016	1,314
Thereafter	2,300
Total	13,385
Components of goodwill [Abstract]
Goodwill	349,874	349,874
Goodwill, Activity	0
Accumulated Impairment	(4,824)	(4,824)
Accumulated impairment, Activity	0
Goodwill, net	345,050	345,050	345,915
Goodwill, net, Activity	0
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	32,437	32,437
Accumulated Amortization	(22,496)	(20,918)
Net Carrying Amount	9,941	11,519
Other Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	9,431	9,429
Accumulated Amortization	(5,987)	(5,434)
Net Carrying Amount	$ 3,444	$ 3,995

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.