LEASES (Details) $ in Thousands	12 Months Ended
	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.