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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, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Total |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [1] | $ 266,529 | $ 243,704 |
Total | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [1] | 233,070 | 221,953 |
Total | Level 2 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [1] | 33,459 | 21,751 |
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 |  | 13,441 | 18,625 |
Money Market Accounts / Cash Equivalents | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 13,441 | 18,625 |
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 |  | 147,763 | 128,539 |
Equity Securities | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 147,763 | 128,539 |
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 |  | 66,709 | 52,682 |
U.S. Large-cap | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 66,709 | 52,682 |
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 |  | 23,127 | 16,432 |
U.S mid/small cap | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 23,127 | 16,432 |
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 | 6,427 |
CBU common stock | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 5,691 | 6,427 |
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 |  | 50,270 | 51,725 |
International | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 50,270 | 51,725 |
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 |  | 1,966 | 1,273 |
other | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 1,966 | 1,273 |
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 |  | 92,566 | 82,198 |
Fixed Income Securities | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 59,107 | 60,447 |
Fixed Income Securities | Level 2 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 33,459 | 21,751 |
Fixed Income Securities | Level 3 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Treasury and Agency Securities [Member] |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 74,598 | 56,562 |
U.S. Treasury and Agency Securities [Member] | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 59,107 | 49,665 |
U.S. Treasury and Agency Securities [Member] | Level 2 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 15,491 | 6,897 |
U.S. Treasury and Agency Securities [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 |  | 17,968 | 18,502 |
Investment Grade Bonds [Member] | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 0 | 3,648 |
Investment Grade Bonds [Member] | Level 2 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 17,968 | 14,854 |
Investment Grade Bonds [Member] | Level 3 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 0 | 0 |
High Yield [Member] |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [2] |  | 7,134 |
High Yield [Member] | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [2] |  | 7,134 |
High Yield [Member] | Level 2 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [2] |  | 0 |
High Yield [Member] | Level 3 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [2] |  | 0 |
Other Investments [Member] |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [3] | 12,759 | 14,342 |
Other Investments [Member] | Level 1 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [3] | 12,759 | 14,342 |
Other Investments [Member] | Level 2 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [3] | 0 | 0 |
Other Investments [Member] | Level 3 |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets | [3] | 0 | 0 |
Dividends and Interest Receivable [Member] |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 900 | 600 |
Pension Benefits |  |  |  |
Defined Benefit Plan |  |  |  |
Fair value of plan assets |  | 267,462 | 244,332 | $ 290,687
Pension Plans |  |  |  |
Contribution made to defined benefit pension plan by employer |  | $ 4,300 | $ 100 |
[1] | Excludes dividends and interest receivable totaling $0.9 million and $0.6 million at December 31, 2023 and 2022, 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.

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