SUBSEQUENT EVENT (Details) (USD $) In Millions, unless otherwise specified	Jul. 20, 2012 HSBC Branch Agreement [Member] Branch	Sep. 07, 2012 First Niagara Branch Agreement [Member] Branch	Jul. 31, 2013 Purchase and Assumption Agreement [Member] Bank of America Agreement [Member] Branch
Subsequent Event [Line Items]
Number of new branch locations included in acquisition or purchase agreement	16	3	8
Deposits Acquired	$ 697	$ 101	$ 369
Blended Deposit Premium	3.40%	3.10%	2.40%
Loans Acquired	$ 106	$ 54	$ 1

The expected number of new branch locations upon completion of the merger, as a result of the acquisition or purchase agreement.

No definition available.

Name:	cbu_NumberOfNewBranchLocationsIncludedInAcquisionOrAgreement
Namespace Prefix:	cbu_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant