Annual report pursuant to Section 13 and 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)

v3.8.0.1
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
T
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)   $ (9.2)
Proved Developed and Undeveloped Reserves, Net, Period Increase (Decrease) | T   115,000,000
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent $ 200.1 $ 206.8
Accrual for Environmental Loss Contingencies 2.9 2.8
Mine Reclamation and Closing Liability, current and noncurrent 197.2 204.0
U.S. Iron Ore [Member]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) (26.2)  
Asia Pacific Iron Ore [Member]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies, Period Increase (Decrease) 10.1  
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member]    
Site Contingency [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent $ 28.6 25.5
Selenium [Domain]    
Site Contingency [Line Items]    
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)   $ (29.6)