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]
Noncontrolling Interest, Ownership Percentage by Parent	100.00%
Partnership distribution payable	$ 44.2	$ 44.2	$ 44.2
U.S. Iron Ore [Member]
Segment Reporting Information [Line Items]
Number of mines (in number of facilities) | Facility	4
Joint Venture Partners [Member] | U.S. Iron Ore [Member]
Segment Reporting Information [Line Items]
Number of mines (in number of facilities) | Facility	1
Tilden [Member]
Segment Reporting Information [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	100.00%			85.00%
Empire [Member]
Segment Reporting Information [Line Items]
Purchase of Noncontrolling Interest	$ 132.7
Noncontrolling Interest, Ownership Percentage by Parent	100.00%			79.00%
Empire [Member] | Other Noncurrent Liabilities [Member]
Segment Reporting Information [Line Items]
Noncontrolling Interest Purchase, Installment Amount	$ 44.2