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BENEFIT PLANS - Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  |  | $ 1,400 |
Total [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | $ 290,247 | 270,953 |
Total [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | 284,137 | 258,483 |
Total [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | 6,110 | 12,470 |
Total [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | 0 | 0 |
Money Market Accounts / Cash Equivalents [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 19,605 | 68,137 |
Money Market Accounts / Cash Equivalents [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 19,605 | 68,137 |
Money Market Accounts / Cash Equivalents [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Money Market Accounts / Cash Equivalents [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Equity Securities [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 169,818 | 143,779 |
Equity Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 169,818 | 137,691 |
Equity Securities [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 6,088 |
Equity Securities [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Large-Cap [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 75,180 | 69,195 |
U.S. Large-Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 75,180 | 69,195 |
U.S. Large-Cap [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Large-Cap [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S Mid/Small Cap [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 12,052 | 17,338 |
U.S Mid/Small Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 12,052 | 17,338 |
U.S Mid/Small Cap [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S Mid/Small Cap [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
CBU common stock [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 7,604 | 7,701 |
CBU common stock [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 7,604 | 7,701 |
CBU common stock [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
CBU common stock [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
International [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 73,891 | 49,545 |
International [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 73,891 | 43,457 |
International [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 6,088 |
International [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Other |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 1,091 |  |
Other | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 1,091 |  |
Other | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 |  |
Other | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 |  |
Fixed Income Securities [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 76,097 | 41,424 |
Fixed Income Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 69,987 | 35,042 |
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 6,110 | 6,382 |
Fixed Income Securities [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Treasury and Agency Securities |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 25,962 | 12,395 |
U.S. Treasury and Agency Securities | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 19,988 | 6,154 |
U.S. Treasury and Agency Securities | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 5,974 | 6,241 |
U.S. Treasury and Agency Securities | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Investment Grade Bonds [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 41,510 | 23,226 |
Investment Grade Bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 41,374 | 23,085 |
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 136 | 141 |
Investment Grade Bonds [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
High Yield [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [2] | 8,625 | 5,803 |
High Yield [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [2] | 8,625 | 5,803 |
High Yield [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [2] | 0 | 0 |
High Yield [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [2] | 0 | 0 |
Other Investments [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 24,727 | 17,613 |
Other Investments [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 24,727 | 17,613 |
Other Investments [Member] | Significant Observable Inputs, Level 2 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 0 | 0 |
Other Investments [Member] | Significant Unobservable Inputs, Level 3 |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 0 | 0 |
Dividends and Interest Receivable [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 400 | 200 |
Pension Benefits [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 290,687 | 272,600 | $ 227,323
Pension Plans [Abstract] |  |  |  |
Contribution made to defined benefit pension plan by employer |  | 3,639 | 7,993 |
Pension Benefits [Member] | Unfunded Plan Member |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 3,500 |  |
Pension Benefits [Member] | Nonqualified Plan [Member] | Unfunded Plan Member |  |  |  |
Pension Plans [Abstract] |  |  |  |
Contribution made to defined benefit pension plan by employer |  | $ 2,900 | $ 3,900 |
[1] | Excludes dividends and interest receivable totaling $0.4 million and $0.2 million at December 31, 2021 and 2020, 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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