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Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v3.19.1
RELATED PARTIES (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Facility
Dec. 31, 2017
USD ($)
Dec. 31, 2018
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý Ìý
Mining and Pelletizing [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility 4 Ìý Ìý
Empire Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent Ìý 100.00% Ìý
Purchase of Noncontrolling Interest | $ Ìý $ 132.7 Ìý
Other Liabilities [Member] | Empire Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount | $ $ 44.2 Ìý Ìý
Hibbing [Member] | Mining and Pelletizing [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility 1 Ìý Ìý
Hibbing [Member] | Investments in Ventures [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Ownership interest, equity method investment 23.00% Ìý 23.00%