ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Loss Contingencies [Line Items]
Environmental	$ 4.3	$ 5.5
Mine Reclamation and Closing Liability, current and noncurrent	211.0	165.3
Total environmental and mine closure obligations	215.3	170.8
Less current portion	3.4	5.2
Long term environmental and mine closure obligations	211.9	165.6
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	171.5	120.9
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	15.6	21.5
LTV Steel Mining Company [Member] | Previously Owned Or Operating Facilities [Member]
Loss Contingencies [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	$ 23.9	$ 22.9