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BENEFIT PLANS - Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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] | 270,953 | $ 226,835 |
Total [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | 258,483 | 213,037 |
Total [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | 12,470 | 13,798 |
Total [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
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 |  | 68,137 | 3,983 |
Money Market Accounts / Cash Equivalents [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 68,137 | 3,983 |
Money Market Accounts / Cash Equivalents [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
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 [Member] |  |  |  |
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 |  | 143,779 | 103,026 |
Equity Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 137,691 | 97,651 |
Equity Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 6,088 | 5,375 |
Equity Securities [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
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 |  | 69,195 | 50,301 |
U.S. Large-Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 69,195 | 50,301 |
U.S. Large-Cap [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
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 [Member] |  |  |  |
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 |  | 17,338 | 10,408 |
U.S Mid/Small Cap [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 17,338 | 10,408 |
U.S Mid/Small Cap [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
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 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
CBU Stock [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 7,701 | 6,522 |
CBU Stock [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 7,701 | 6,522 |
CBU Stock [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
CBU Stock [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
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 |  | 49,545 | 35,795 |
International [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 43,457 | 30,420 |
International [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 6,088 | 5,375 |
International [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
Fixed Income Securities [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 41,424 | 100,256 |
Fixed Income Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 35,042 | 91,915 |
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 6,382 | 8,341 |
Fixed Income Securities [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
U.S. Treasury and Agency Securities [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 12,395 | 82,039 |
U.S. Treasury and Agency Securities [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 6,154 | 73,698 |
U.S. Treasury and Agency Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 6,241 | 8,341 |
U.S. Treasury and Agency Securities [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
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 |  | 23,226 | 12,938 |
Investment Grade Bonds [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 23,085 | 12,938 |
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 141 | 0 |
Investment Grade Bonds [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
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] | 5,803 | 5,279 |
High Yield [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [2] | 5,803 | 5,279 |
High Yield [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
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 [Member] |  |  |  |
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] | 17,613 | 19,570 |
Other Investments [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 17,613 | 19,488 |
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 0 | 82 |
Other Investments [Member] | Significant Unobservable Inputs, Level 3 [Member] |  |  |  |
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 |  | 200 | 500 |
Pension Benefits [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 272,600 | 227,323 | $ 203,672
Pension Plans [Abstract] |  |  |  |
Contribution made to defined benefit pension plan by employer |  | 7,993 | 7,893 |
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 |  | $ 3,900 | $ 7,300 |
[1] | Excludes dividends and interest receivable totaling $0.2 million and $0.5 million at December 31, 2020 and 2019, 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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