LOANS, Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Allowance for loan losses [Roll Forward]
Beginning balance	$ 45,005	$ 44,197	$ 45,341	$ 44,319
Charge-offs	(2,288)	(2,857)	(4,702)	(5,205)
Recoveries	1,974	1,375	3,429	2,601
Provision	591	1,900	1,214	2,900
Ending balance	45,282	44,615	45,282	44,615
Acquired Impaired [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	196	201	173	530
Charge-offs	(43)	(7)	(43)	(20)
Recoveries	0	0	0	0
Provision	(60)	(31)	(37)	(347)
Ending balance	93	163	93	163
Residential Portfolio Segment [Member] \| Consumer Mortgage [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	10,233	9,281	10,286	8,994
Charge-offs	(199)	(364)	(642)	(531)
Recoveries	45	12	66	53
Provision	113	446	482	859
Ending balance	10,192	9,375	10,192	9,375
Commercial Portfolio Segment [Member] \| Business Lending [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	15,405	17,046	15,787	17,507
Charge-offs	(299)	(385)	(433)	(505)
Recoveries	527	100	608	271
Provision	(280)	(208)	(609)	(720)
Ending balance	15,353	16,553	15,353	16,553
Consumer Portfolio Segment [Member] \| Consumer Indirect [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	11,246	10,586	11,544	10,248
Charge-offs	(1,397)	(1,435)	(2,823)	(2,862)
Recoveries	1,184	1,005	2,337	1,801
Provision	569	1,198	544	2,167
Ending balance	11,602	11,354	11,602	11,354
Consumer Portfolio Segment [Member] \| Consumer Direct [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	2,879	3,087	3,083	3,181
Charge-offs	(294)	(420)	(639)	(912)
Recoveries	199	226	392	438
Provision	207	405	155	591
Ending balance	2,991	3,298	2,991	3,298
Consumer Portfolio Segment [Member] \| Home Equity [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	2,663	1,818	2,701	1,830
Charge-offs	(56)	(246)	(122)	(375)
Recoveries	19	32	26	38
Provision	51	256	72	367
Ending balance	2,677	1,860	2,677	1,860
Unallocated Financing Receivables [Member]
Allowance for loan losses [Roll Forward]
Beginning balance	2,383	2,178	1,767	2,029
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(9)	(166)	607	(17)
Ending balance	$ 2,374	$ 2,012	$ 2,374	$ 2,012

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(1)(ii)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

Name:	us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=cbu_AcquiredImpairedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_ConsumerInstallmentIndirectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_ConsumerInstallmentDirectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cbu_HomeEquityReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: