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EMPLOYEE BENEFIT PLANS (Nonunion Plan Assets) (Details) - Nonunion Defined Benefit Pension Plan | 6 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Jun. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans |  |  |  |  |  |
Period over which the possibility of experiencing a substantial loss is limited by adequate diversification under the long-term asset allocation policy |  |  |  | 1 year |  |
Expected return on plan assets (as a percent) | 2.50% | 6.50% |  |  | 6.50% | 6.50%
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 100.00% |  |  | 100.00% |  |
Weighted-Average Allocation (as a percent) | 100.00% |  |  | 100.00% | 100.00% |
Maximum performance period of investment fund used to compare investment performance |  |  |  | 3 years |  |
Minimum performance period of total fund used to compare investment performance |  |  |  | 5 years |  |
Forecast |  |  |  |  |  |
Nonunion Defined Benefit Pension, Supplemental Benefit, and Postretirement Health Benefit Plans |  |  |  |  |  |
Expected return on plan assets (as a percent) |  |  | 1.40% |  |  |
Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Performance period of recognized market indices used to compare investment performance |  |  |  | 3 years |  |
Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Performance period of recognized market indices used to compare investment performance |  |  |  | 5 years |  |
Large Cap U.S. Equity |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Weighted-Average Allocation (as a percent) | 0.00% |  |  | 0.00% | 14.00% |
Large Cap U.S. Equity | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Large Cap U.S. Equity | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 20.00% |  |  | 20.00% |  |
Mid Cap U.S. Equity |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Weighted-Average Allocation (as a percent) | 0.00% |  |  | 0.00% | 9.40% |
Mid Cap U.S. Equity | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Mid Cap U.S. Equity | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 11.00% |  |  | 11.00% |  |
Small Cap U.S. Equity |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Weighted-Average Allocation (as a percent) | 0.00% |  |  | 0.00% | 10.00% |
Small Cap U.S. Equity | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Small Cap U.S. Equity | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 11.00% |  |  | 11.00% |  |
International Equity |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Weighted-Average Allocation (as a percent) | 0.00% |  |  | 0.00% | 14.40% |
International Equity | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
International Equity | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 18.00% |  |  | 18.00% |  |
Debt Instruments |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 70.00% |  |  | 70.00% |  |
Weighted-Average Allocation (as a percent) | 73.60% |  |  | 73.60% | 25.00% |
Debt Instruments | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 20.00% |  |  | 20.00% |  |
Debt Instruments | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 100.00% |  |  | 100.00% |  |
Actively managed portfolio of short-term debt instruments | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Maturity period of investments |  |  |  | 1 year |  |
Actively managed portfolio of short-term debt instruments | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Maturity period of investments |  |  |  | 5 years |  |
Actively managed portfolio of short-duration debt instruments | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Maturity period of investments |  |  |  | 1 year |  |
Actively managed portfolio of short-duration debt instruments | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Maturity period of investments |  |  |  | 3 years |  |
Floating Rate Loan Fund |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 10.00% |  |  | 10.00% |  |
Weighted-Average Allocation (as a percent) | 13.10% |  |  | 13.10% | 10.80% |
Floating Rate Loan Fund | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 3.00% |  |  | 3.00% |  |
Floating Rate Loan Fund | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 100.00% |  |  | 100.00% |  |
Cash and Cash Equivalents |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 20.00% |  |  | 20.00% |  |
Weighted-Average Allocation (as a percent) | 13.30% |  |  | 13.30% | 16.40% |
Cash and Cash Equivalents | Minimum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 0.00% |  |  | 0.00% |  |
Cash and Cash Equivalents | Maximum |  |  |  |  |  |
Target allocations, acceptable ranges, and actual asset allocations |  |  |  |  |  |
Target allocation (as a percent) | 100.00% |  |  | 100.00% |  |

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