ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Environmental	$ 2.8	$ 2.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current	12.8	12.9
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent	198.2	193.9
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	190.9	187.8
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	17.3	16.2
North American Coal [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	208.2	204.0
Eastern Canadian Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	$ 211.0	$ 206.8