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Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v3.8.0.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2018
USD ($)
Facility
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Mar. 31, 2017
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý Ìý Ìý
Partnership distribution payable $ 44.2 $ 44.2 $ 44.2 Ìý
U.S. Iron Ore [Member] Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility 4 Ìý Ìý Ìý
Joint Venture Partners [Member] | U.S. Iron Ore [Member] Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility 1 Ìý Ìý Ìý
Tilden [Member] Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý Ìý 85.00%
Empire [Member] Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý
Purchase of Noncontrolling Interest $ 132.7 Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý Ìý 79.00%
Empire [Member] | Other Noncurrent Liabilities [Member] Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount $ 44.2 Ìý Ìý Ìý