Quarterly report pursuant to Section 13 or 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details)

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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Loss Contingencies [Line Items]    
Environmental $ 3.2 $ 2.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent 176.0 171.3
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current 3.7 3.6
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent 172.3 167.7
Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent $ 172.8 $ 168.4