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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017 | Dec. 31, 2016
Empire [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Increase in Ownership Equity | 21.00% |  |
Purchase of Noncontrolling Interest | $ 132.7 |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% | 100.00% | 79.00%
Noncontrolling Interest Purchase, Installment Amount | $ 44.2 |  |
Stockholders' Equity, Period Increase (Decrease) | 16.0 |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests | $ 116.7 | $ (132.7) |
Tilden [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Increase in Ownership Equity | 15.00% |  |
Purchase of Noncontrolling Interest | $ 105.0 |  |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00% | 100.00% | 85.00%
Stockholders' Equity, Period Increase (Decrease) | $ 89.1 |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests | 15.9 | $ (105.0) |
Other Noncurrent Liabilities [Member] | Empire [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Noncontrolling Interest Purchase, Installment Amount | $ 44.2 |  |
Hibbing [Member] | Other Noncurrent Assets [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Ownership interest, equity method investment | 23.00% | 23.00% | 23.00%
Hibbing [Member] | Other Noncurrent Liabilities [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Equity Method Investments | $ 6.1 | $ 6.1 | $ 8.7

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