RELATED PARTIES (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2017	Dec. 31, 2018
Segment Reporting Information [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	100.00%
Empire
Segment Reporting Information [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent		100.00%
Purchase of Noncontrolling Interest		$ 132.7
Other Liabilities [Member] | Empire
Segment Reporting Information [Line Items]
Noncontrolling Interest Purchase, Installment Amount	$ 44.2
Hibbing [Member]
Segment Reporting Information [Line Items]
Ownership interest, equity method investment	23.00%		23.00%