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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 9 Months Ended
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
Facility
Sep. 30, 2016
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
Mar. 31, 2016
USD ($)
U.S. Iron Ore [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility Ìý Ìý 4 Ìý Ìý Ìý Ìý
Joint Venture Partners [Member] | U.S. Iron Ore [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Number of mines (in number of facilities) | Facility Ìý Ìý 1 Ìý Ìý Ìý Ìý
Tilden [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase in Ownership Equity 15.00% Ìý Ìý Ìý Ìý Ìý Ìý
Purchase of Noncontrolling Interest $ 105.0 Ìý Ìý Ìý Ìý Ìý Ìý
Stockholders' Equity, Period Increase (Decrease) 89.1 Ìý Ìý Ìý Ìý Ìý Ìý
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ 15.9 Ìý $ (105.0) Ìý Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý 100.00% Ìý Ìý 85.00% Ìý
Empire [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Increase in Ownership Equity 21.00% Ìý Ìý Ìý Ìý Ìý Ìý
Purchase of Noncontrolling Interest $ 132.7 Ìý Ìý Ìý Ìý Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount 44.2 Ìý Ìý Ìý Ìý Ìý Ìý
Stockholders' Equity, Period Increase (Decrease) 16.0 Ìý Ìý Ìý Ìý Ìý Ìý
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests $ 116.7 Ìý $ (132.7) Ìý Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Parent 100.00% Ìý 100.00% Ìý Ìý 79.00% Ìý
Empire [Member] | Other Current Liabilities [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount $ 44.2 Ìý Ìý Ìý Ìý Ìý Ìý
Empire [Member] | Other Noncurrent Liabilities [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Noncontrolling Interest Purchase, Installment Amount $ 44.2 Ìý Ìý Ìý Ìý Ìý Ìý
Empire [Member] | Arcelor Mittal [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Subsequent Paid Distributions of Partners' Equity Amounts Ìý Ìý Ìý Ìý $ 8.7 Ìý $ 11.1
Recorded Distributions of Partners' Equity Amounts Ìý $ 7.4 Ìý $ 48.8 Ìý Ìý Ìý
Paid Distributions of Partners' Equity Amounts Ìý $ 41.4 Ìý $ 41.4 Ìý Ìý Ìý