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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, 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.

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