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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
T
Mar. 31, 2017
USD ($)
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) $ 9.2  
Reserve increase (decrease), tons | T 115,000,000  
Selenium [Domain]    
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) $ (29.6)  
Eastern Canadian Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Mine Reclamation and Closing Liability, current and noncurrent $ 206.8 $ 211.0