PENSION AND OTHER BENEFIT PLANS - Fair value of plan assets by assets category (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Total
Defined Benefit Plan
Fair value of plan assets	[1]	$ 243,704	$ 290,247
Total | Level 1
Defined Benefit Plan
Fair value of plan assets	[1]	226,599	284,137
Total | Level 2
Defined Benefit Plan
Fair value of plan assets	[1]	17,105	6,110
Total | Level 3
Defined Benefit Plan
Fair value of plan assets	[1]	0	0
Money Market Accounts / Cash Equivalents
Defined Benefit Plan
Fair value of plan assets		18,625	19,605
Money Market Accounts / Cash Equivalents | Level 1
Defined Benefit Plan
Fair value of plan assets		18,625	19,605
Money Market Accounts / Cash Equivalents | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
Money Market Accounts / Cash Equivalents | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
Equity Securities
Defined Benefit Plan
Fair value of plan assets		128,539	169,818
Equity Securities | Level 1
Defined Benefit Plan
Fair value of plan assets		128,539	169,818
Equity Securities | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
Equity Securities | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
U.S. Large-Cap
Defined Benefit Plan
Fair value of plan assets		52,682	75,180
U.S. Large-Cap | Level 1
Defined Benefit Plan
Fair value of plan assets		52,682	75,180
U.S. Large-Cap | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
U.S. Large-Cap | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
U.S. mid/small cap
Defined Benefit Plan
Fair value of plan assets		16,432	12,052
U.S. mid/small cap | Level 1
Defined Benefit Plan
Fair value of plan assets		16,432	12,052
U.S. mid/small cap | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
U.S. mid/small cap | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
CBU common stock
Defined Benefit Plan
Fair value of plan assets		6,427	7,604
CBU common stock | Level 1
Defined Benefit Plan
Fair value of plan assets		6,427	7,604
CBU common stock | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
CBU common stock | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
International
Defined Benefit Plan
Fair value of plan assets		51,725	73,891
International | Level 1
Defined Benefit Plan
Fair value of plan assets		51,725	73,891
International | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
International | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
other
Defined Benefit Plan
Fair value of plan assets		1,273	1,091
other | Level 1
Defined Benefit Plan
Fair value of plan assets		1,273	1,091
other | Level 2
Defined Benefit Plan
Fair value of plan assets		0	0
other | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
Fixed Income Securities
Defined Benefit Plan
Fair value of plan assets		82,198	76,097
Fixed Income Securities | Level 1
Defined Benefit Plan
Fair value of plan assets		65,093	69,987
Fixed Income Securities | Level 2
Defined Benefit Plan
Fair value of plan assets		17,105	6,110
Fixed Income Securities | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
US Treasury And Government
Defined Benefit Plan
Fair value of plan assets		56,562	25,962
US Treasury And Government | Level 1
Defined Benefit Plan
Fair value of plan assets		44,995	19,988
US Treasury And Government | Level 2
Defined Benefit Plan
Fair value of plan assets		11,567	5,974
US Treasury And Government | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
Investment Grade Bonds
Defined Benefit Plan
Fair value of plan assets		18,502	41,510
Investment Grade Bonds | Level 1
Defined Benefit Plan
Fair value of plan assets		12,964	41,374
Investment Grade Bonds | Level 2
Defined Benefit Plan
Fair value of plan assets		5,538	136
Investment Grade Bonds | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
High Yield
Defined Benefit Plan
Fair value of plan assets	[2]	7,134	8,625
High Yield | Level 1
Defined Benefit Plan
Fair value of plan assets	[2]	7,134	8,625
High Yield | Level 2
Defined Benefit Plan
Fair value of plan assets	[2]	0	0
High Yield | Level 3
Defined Benefit Plan
Fair value of plan assets	[2]	0	0
Other Investments
Defined Benefit Plan
Fair value of plan assets	[3]	14,342	24,727
Other Investments | Level 1
Defined Benefit Plan
Fair value of plan assets	[3]	14,342	24,727
Other Investments | Level 2
Defined Benefit Plan
Fair value of plan assets	[3]	0	0
Other Investments | Level 3
Defined Benefit Plan
Fair value of plan assets	[3]	0	0
Dividends and Interest Receivable
Defined Benefit Plan
Fair value of plan assets		600	400
Pension Benefits
Defined Benefit Plan
Fair value of plan assets		244,332	290,687	$ 272,600
Pension Plans
Contribution made to defined benefit pension plan by employer		906	3,639
Pension Benefits | Nonqualified Plan | Unfunded Plan
Pension Plans
Contribution made to defined benefit pension plan by employer		$ 100	$ 2,900

[1] Excludes dividends and interest receivable totaling $0.6 million and $0.4 million at December 31, 2022 and 2021, respectively.

[2] This category is exchange-traded funds representing a diversified index of high yield corporate bonds.

[3] This category is comprised of exchange-traded funds and mutual funds holding non-traditional investment classes including private equity funds and alternative exchange funds.