Quarterly report pursuant to Section 13 or 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)

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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Jun. 30, 2018
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent $ 171.3 $ 176.0
U.S. Iron Ore [Member]    
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) $ (26.2)