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EMPLOYEE BENEFIT PLANS (Plans) (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) | Mar. 31, 2019 USD ($) | Mar. 31, 2017 USD ($) person | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans |  |  |  |  |  |
Pension settlement expense, pre-tax |  |  |  | $ 12,925 | $ 4,156 | $ 3,229
Nonunion Defined Benefit Pension Plan |  |  |  |  |  |
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans |  |  |  |  |  |
Nonparticipating annuity contract |  |  | $ 7,600 |  | 7,600 |
Pension settlement expense, pre-tax |  |  |  | 12,925 | 4,156 | $ 3,023
Employer contributions | $ 5,500 |  |  | 5,500 |  |
Number of plan participants for which vested pension benefits were settled | person |  |  | 50 |  |  |
Postretirement Health Benefit Plan |  |  |  |  |  |
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans |  |  |  |  |  |
Employer contributions |  |  |  | $ 769 | $ 733 |
Unrecognized prior service credit, amortization period |  |  |  | 9 years |  |
Forecast | Nonunion Defined Benefit Pension Plan |  |  |  |  |  |
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans |  |  |  |  |  |
Pension settlement expense, pre-tax |  | $ 4,000 |  |  |  |
Pension settlement expense, after-tax |  | 3,000 |  |  |  |
Employer contributions |  | $ 7,000 |  |  |  |

X
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