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BENEFIT PLANS, Fair Value of Plan Assets by Assets Category (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Total [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | $ 226,835 | $ 202,943 |
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] | 218,412 | 191,507 |
Total [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [1] | 8,423 | 11,436 |
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 |  | 3,983 | 4,856 |
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 |  | 3,983 | 4,856 |
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 |  | 105,221 | 89,201 |
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 |  | 105,221 | 89,201 |
Equity Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
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 |  | 50,301 | 39,122 |
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 |  | 50,301 | 39,122 |
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 |  | 10,408 | 9,881 |
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 |  | 10,408 | 9,881 |
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 |  | 6,522 | 7,692 |
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 |  | 6,522 | 7,692 |
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 |  | 37,990 | 32,506 |
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 |  | 37,990 | 32,506 |
International [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 0 |
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 |  | 101,693 | 94,553 |
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 |  | 93,352 | 83,183 |
Fixed Income Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 8,341 | 11,370 |
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 |
Government Securities [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 82,039 | 75,787 |
Government 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 |  | 73,698 | 64,417 |
Government Securities [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 8,341 | 11,370 |
Government 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 |  | 12,884 | 12,054 |
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 |  | 12,884 | 12,054 |
Investment Grade Bonds [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 0 | 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] | 6,770 | 6,712 |
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] | 6,770 | 6,712 |
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] | 15,938 | 14,333 |
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] | 15,856 | 14,267 |
Other Investments [Member] | Significant Observable Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets | [3] | 82 | 66 |
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 |  | 500 | 700 |
Pension Benefits [Member] |  |  |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Fair value of plan assets |  | 227,323 | 203,672 | $ 217,107
Pension Plans [Abstract] |  |  |  |
Contribution made to defined benefit pension plan by employer |  | 7,893 | $ 605 |
Pension Benefits [Member] | Nonqualified Plan [Member] | us-gaap_UnfundedPlanMember |  |  |  |
Pension Plans [Abstract] |  |  |  |
Contribution made to defined benefit pension plan by employer |  | $ 7,300 |  |
[1] | Excludes dividends and interest receivable totaling $0.5 million and $0.7 million at December 31, 2019 and 2018, 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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