ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Environmental	$ 2.8	$ 3.6
Mine Reclamation and Closing Liability, current and noncurrent	204.0	230.4
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent	206.8	234.0
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current	12.9	2.8
Environmental Loss Contingency And Mine Reclamation And Closing Liability Noncurrent	193.9	231.2
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	187.8	214.0
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	16.2	16.4
LTV Steel Mining Company [Member] | Previously Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	$ 25.5	$ 24.1