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Quarterly report pursuant to Section 13 or 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

v3.8.0.1
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] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Increase in Ownership Equity 21.00% Ìý Ìý
Purchase of Noncontrolling Interest $ 132.7 Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% 100.00% 79.00%
Noncontrolling Interest Purchase, Installment Amount $ 44.2 Ìý Ìý
Stockholders' Equity, Period Increase (Decrease) 16.0 Ìý Ìý
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ 116.7 $ (132.7) Ìý
Tilden [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Increase in Ownership Equity 15.00% Ìý Ìý
Purchase of Noncontrolling Interest $ 105.0 Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% 100.00% 85.00%
Stockholders' Equity, Period Increase (Decrease) $ 89.1 Ìý Ìý
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests 15.9 $ (105.0) Ìý
Other Noncurrent Liabilities [Member] | Empire [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount $ 44.2 Ìý Ìý
Hibbing [Member] | Other Noncurrent Assets [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Ownership interest, equity method investment 23.00% 23.00% 23.00%
Hibbing [Member] | Other Noncurrent Liabilities [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Equity Method Investments $ 6.1 $ 6.1 $ 8.7