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PENSION AND OTHER BENEFIT PLANS - Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension Plans
Fair Value		$ 6,431
Unfunded Commitments		0
Total
Defined Benefit Plan
Fair value of plan assets	[1]	271,176	$ 266,529
Total | Level 1
Defined Benefit Plan
Fair value of plan assets	[1]	175,417	233,070
Total | Level 2
Defined Benefit Plan
Fair value of plan assets	[1]	87,811	33,459
Total | Level 3
Defined Benefit Plan
Fair value of plan assets	[1]	1,517	0
Money Market Accounts / Cash Equivalents
Defined Benefit Plan
Fair value of plan assets		1,526	13,441
Money Market Accounts / Cash Equivalents | Level 1
Defined Benefit Plan
Fair value of plan assets		1,526	13,441
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		114,598	147,763
Equity Securities | Level 1
Defined Benefit Plan
Fair value of plan assets		114,598	147,763
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		62,289	66,709
U.S. Large-cap | Level 1
Defined Benefit Plan
Fair value of plan assets		62,289	66,709
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		17,048	23,127
U.S mid/small cap | Level 1
Defined Benefit Plan
Fair value of plan assets		17,048	23,127
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			5,691
CBU common stock | Level 1
Defined Benefit Plan
Fair value of plan assets			5,691
CBU common stock | Level 2
Defined Benefit Plan
Fair value of plan assets			0
CBU common stock | Level 3
Defined Benefit Plan
Fair value of plan assets			0
International
Defined Benefit Plan
Fair value of plan assets		34,074	50,270
International | Level 1
Defined Benefit Plan
Fair value of plan assets		34,074	50,270
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		1,187	1,966
other | Level 1
Defined Benefit Plan
Fair value of plan assets		1,187	1,966
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		144,382	92,566
Fixed Income Securities | Level 1
Defined Benefit Plan
Fair value of plan assets		59,293	59,107
Fixed Income Securities | Level 2
Defined Benefit Plan
Fair value of plan assets		85,089	33,459
Fixed Income Securities | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
US Treasury and Government [Member]
Defined Benefit Plan
Fair value of plan assets		82,804	74,598
US Treasury and Government [Member] | Level 1
Defined Benefit Plan
Fair value of plan assets		59,293	59,107
US Treasury and Government [Member] | Level 2
Defined Benefit Plan
Fair value of plan assets		23,511	15,491
US Treasury and Government [Member] | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
Investment Grade Bonds [Member]
Defined Benefit Plan
Fair value of plan assets		61,578	17,968
Investment Grade Bonds [Member] | Level 1
Defined Benefit Plan
Fair value of plan assets		0	0
Investment Grade Bonds [Member] | Level 2
Defined Benefit Plan
Fair value of plan assets		61,578	17,968
Investment Grade Bonds [Member] | Level 3
Defined Benefit Plan
Fair value of plan assets		0	0
Other Investments [Member]
Defined Benefit Plan
Fair value of plan assets		4,239	12,759
Other Investments [Member] | Level 1
Defined Benefit Plan
Fair value of plan assets		0	12,759
Other Investments [Member] | Level 2
Defined Benefit Plan
Fair value of plan assets		2,722	0
Other Investments [Member] | Level 3
Defined Benefit Plan
Fair value of plan assets		1,517	0
Defined benefit plan, Alternative Investments
Defined Benefit Plan
Fair value of plan assets		6,431
Defined benefit plan, Alternative Investments | Level 1
Defined Benefit Plan
Fair value of plan assets		0
Defined benefit plan, Alternative Investments | Level 2
Defined Benefit Plan
Fair value of plan assets		0
Defined benefit plan, Alternative Investments | Level 3
Defined Benefit Plan
Fair value of plan assets		0
Dividends and Interest Receivable [Member]
Defined Benefit Plan
Fair value of plan assets		1,600	900
Pension Benefits
Defined Benefit Plan
Fair value of plan assets		273,024	267,462	$ 244,332
Pension Plans
Contribution made to defined benefit pension plan by employer		$ 4,400	$ 4,300
[1] Excludes dividends and interest receivable totaling $1.6 million and $0.9 million at December 31, 2024 and 2023, respectively.

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