ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Environmental	$ 2.9	$ 2.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current	10.8	12.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent	205.4	193.9
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	195.0	187.8
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	18.3	16.2
North American Coal [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	213.3	204.0
Eastern Canadian Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	$ 216.2	$ 206.8