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EMPLOYEE BENEFIT PLANS (Discount rate and assumptions) (Details) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2017 | Jun. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Nonunion Defined Benefit Pension Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average assumptions used to determine nonunion benefit obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) | 3.90% |  |  | 3.10% |  |  |  |  |  | 3.10% |  | 3.90% | 3.10% |
Weighted-average assumptions used to determine net periodic benefit cost |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) | 3.60% | 3.80% | 3.60% | 3.10% | 3.20% | 3.40% | 2.70% | 2.70% | 3.00% |  |  | 3.10% | 3.40% | 3.50%
Expected return on plan assets, net of estimated expenses (as a percent) |  |  |  |  |  |  |  |  |  | 2.50% | 6.50% | 1.40% | 6.50% | 6.50%
Supplemental Benefit Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average assumptions used to determine nonunion benefit obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) | 3.60% |  |  | 2.80% |  |  |  |  |  | 2.80% |  | 3.60% | 2.80% |
Weighted-average assumptions used to determine net periodic benefit cost |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 2.80% | 2.70% | 2.60%
Postretirement Health Benefit Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average assumptions used to determine nonunion benefit obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) | 4.20% |  |  | 3.50% |  |  |  |  |  | 3.50% |  | 4.20% | 3.50% |
Weighted-average assumptions used to determine net periodic benefit cost |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 3.50% | 4.00% | 4.20%

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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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