BORROWINGS, Contractual Maturity on Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Carrying value	$ 248,370	$ 403,446
Weighted-average rate at period end	1.70%
Weighted-average interest rate on borrowings during period	1.46%	0.82%
Federal Home Loan Bank Overnight Advance [Member]
Debt Instrument [Line Items]
Carrying value	$ 146,200	$ 301,300
Federal Home Loan Bank Overnight Advance [Member] \| January 3, 2017 [Member]
Debt Instrument [Line Items]
Carrying value	$ 146,200
Weighted-average rate at period end	0.74%
Subordinated Debt Held By Unconsolidated Subsidiary Trust [Member]
Debt Instrument [Line Items]
Carrying value	$ 102,170	$ 102,146
Subordinated Debt Held By Unconsolidated Subsidiary Trust [Member] \| July 31, 2031 [Member]
Debt Instrument [Line Items]
Carrying value	$ 24,850
Weighted-average rate at period end	4.47%
Subordinated Debt Held By Unconsolidated Subsidiary Trust [Member] \| December 15, 2036 [Member]
Debt Instrument [Line Items]
Carrying value	$ 77,320
Weighted-average rate at period end	2.61%

The average effective interest rate during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount of long-term debt and capital lease obligations including current maturities.

+ References

No definition available.

+ Details

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

- Definition

Weighted average interest rate of long-term debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_LongtermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_FederalHomeLoanBankAdvancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cbu_TotalDebtMaturingOnJanuary32017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=cbu_SubordinatedDebtHeldByUnconsolidatedSubsidiaryTrustMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cbu_TotalDebtMaturingOnJuly312031Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cbu_TotalDebtMaturingOnDecember152036Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: