ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Environmental	$ 2.7	$ 2.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent	177.3	171.3
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current	2.9	3.6
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent	174.4	167.7
Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	$ 174.6	$ 168.4