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Annual report pursuant to Section 13 and 15(d)

RELATED PARTIES (Narrative) (Details)

v3.8.0.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Facility
Dec. 31, 2016
USD ($)
Segment Reporting Information [Line Items] Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý
Due to Related Parties, Current $ 12.3 $ 0.0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ (105.0) Ìý
U.S. Iron Ore [Member] Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý
Number Of Mines | Facility 4 Ìý
Joint Venture Partners [Member] | U.S. Iron Ore [Member] Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý
Number Of Mines | Facility 1 Ìý
Empire [Member] Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý
Increase in Ownership Equity 21.00% Ìý
Purchase of Noncontrolling Interest $ 132.7 Ìý
Stockholders' Equity, Period Increase (Decrease) (12.1) Ìý
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests (116.7) Ìý
Empire [Member] | Arcelor Mittal [Member] Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý
Recorded Distributions of Partners' Equity Amounts 57.5 51.7
Paid Distributions of Partners' Equity Amounts 48.8 40.6
Subsequent Paid Distributions of Partners' Equity Amounts 8.7 $ 11.1
Other Liabilities [Member] | Empire [Member] Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount 44.2 Ìý
Other Noncurrent Liabilities [Member] | Empire [Member] Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount $ 44.2 Ìý