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EMPLOYEE BENEFIT PLANS (Future benefit payments) (Details) - USD ($) $ in Thousands | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018
Nonunion Defined Benefit Pension Plan |  |  |  |
Estimated future benefit payments |  |  |  |
2019 |  | $ 33,373 |  |
Employer contributions | $ 5,500 | 5,500 |  |
Adjusted funding target attainment percentage |  |  |  | 109.30%
Supplemental Benefit Plan |  |  |  |
Estimated future benefit payments |  |  |  |
2019 |  | 938 |  |
2020 |  | 2,169 |  |
2024-2028 |  | 1,142 |  |
Employer contributions |  |  | $ 989 |
Postretirement Health Benefit Plan |  |  |  |
Estimated future benefit payments |  |  |  |
2019 |  | 995 |  |
2020 |  | 1,046 |  |
2021 |  | 1,092 |  |
2022 |  | 1,177 |  |
2023 |  | 1,316 |  |
2024-2028 |  | 7,031 |  |
Employer contributions |  | $ 769 | $ 733 |

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