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PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Net Benefit Cost) (Details) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Pension Benefits [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 8.25% | 8.25% | 8.25%
Other Benefits [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 7.00% | 7.00% | 7.00%
SERP [Domain] | Pension Benefits [Member] | Obligation [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.54% | 3.90% | 4.01%
SERP [Domain] | Pension Benefits [Member] | Service Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.43% | 3.69% | 3.87%
SERP [Domain] | Pension Benefits [Member] | Interest Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.16% | 3.36% | 3.30%
Ore Mining [Domain] | Pension Benefits [Member] | Obligation [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.61% | 4.04% | 4.28%
Ore Mining [Domain] | Pension Benefits [Member] | Service Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.75% | 4.27% | 4.60%
Ore Mining [Domain] | Pension Benefits [Member] | Interest Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.22% | 3.41% | 3.48%
Salary [Member] | Pension Benefits [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Rate of compensation increase | 3.00% | 3.00% | 3.00%
Salary [Member] | Pension Benefits [Member] | Obligation [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.52% | 3.91% | 4.13%
Salary [Member] | Pension Benefits [Member] | Service Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.53% | 3.93% | 4.14%
Salary [Member] | Pension Benefits [Member] | Interest Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.08% | 3.21% | 3.21%
Salary [Member] | Other Benefits [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Rate of compensation increase | 3.00% | 3.00% | 3.00%
Salary [Member] | Other Benefits [Member] | Obligation [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.57% | 3.98% | 4.22%
Salary [Member] | Other Benefits [Member] | Service Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.76% | 4.30% | 4.63%
Salary [Member] | Other Benefits [Member] | Interest Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.15% | 3.28% | 3.31%
Hourly [Member] | Pension Benefits [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets | 2.00% | 2.00% | 2.00%
Hourly [Member] | Pension Benefits [Member] | Obligation [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.61% | 4.02% | 4.27%
Hourly [Member] | Pension Benefits [Member] | Service Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.76% | 4.30% | 4.66%
Hourly [Member] | Pension Benefits [Member] | Interest Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.21% | 3.38% | 3.46%
Hourly [Member] | Other Benefits [Member] | Obligation [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.60% | 4.03% | 4.32%
Hourly [Member] | Other Benefits [Member] | Service Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.73% | 4.23% | 4.56%
Hourly [Member] | Other Benefits [Member] | Interest Cost [Domain] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.10% | 3.24% | 3.48%

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- Definition Weighted average rate for present value of future retirement benefits cash flows, 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)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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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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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related 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)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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