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LEASES, Operating Lease That Has Not Yet Commenced (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2019
Operating lease that has not yet commenced [Abstract] |  |  |
Lease term |  |  | 5 years
Right-of-use assets |  |  | $ 0.6
Lease liabilities |  |  | $ 0.6
Rental expense | $ 9.0 | $ 7.3 |

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- Definition Present value of lessee's discounted obligation for lease payments from operating lease that has not yet commenced. + References No definition available. + Details
Name: | cbu_OperatingLeaseLeaseNotYetCommencedLiability
Namespace Prefix: | cbu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of lessee's right to use underlying asset under operating lease that has not yet commenced. + References No definition available. + Details
Name: | cbu_OperatingLeaseLeaseNotYetCommencedRightOfUseAsset
Namespace Prefix: | cbu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Term of lessee's operating lease not yet commenced, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980 + Details
Name: | us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LesseeOperatingLeaseNotYetCommencedDescriptionAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
Name: | us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration