BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Revenue Reimbursement) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Freight and Reimbursement Revenue	$ 160.1	$ 221.4	$ 174.8
Mining and Pelletizing [Member]
Freight and Reimbursement Revenue	160.1	221.4	174.8
Mining and Pelletizing [Member] | Freight Revenue [Member]
Freight and Reimbursement Revenue	160.1	166.7	106.8
Mining and Pelletizing [Member] | Co-venturer [Member]
Freight and Reimbursement Revenue	0.0	$ 54.7	$ 68.0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Freight and Reimbursement Revenue	$ 156.2