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LEASES (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 USD ($)
Operating Leases [Abstract] |  |
Term of option to terminate | 1 year |
Components of Lease Expense [Abstract] |  |
Operating lease cost | $ 8,724 |
Variable lease cost | 18 |
Short-term lease cost | 240 | [1]
Total lease cost | 8,982 |
Cash paid for amounts included in the measurement of lease liabilities [Abstract] |  |
Operating cash outflows for operating leases | 7,938 |
Right-of-use assets obtained in exchange for lease obligations [Abstract] |  |
Operating leases | 14,145 |
Operating leases [Abstract] |  |
Operating lease right-of-use assets | $ 39,895 |
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] | us-gaap:PropertyPlantAndEquipmentNet |
Operating lease liabilities | $ 40,913 |
Operating Lease, Liability, Statement of Financial Position [Extensible List] | cbu:AccruedInterestAndOtherLiabilities |
Weighted average remaining lease term [Abstract] |  |
Operating leases | 6 years 7 months 6 days |
Weighted average discount rate [Abstract] |  |
Operating leases | 2.95% |
Minimum [Member] |  |
Operating Leases [Abstract] |  |
Remaining term of lease | 1 year |
Term of option to extend | 1 year |
Maximum [Member] |  |
Operating Leases [Abstract] |  |
Remaining term of lease | 15 years |
Term of option to extend | 40 years |
[1] | Short-term lease cost includes the cost of leases with terms of twelve months or less, excluding leases with terms of one month or less.

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