Environmental And Mine Closure Obligations (Summary Of Mine Closure Obligations) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Loss Contingencies [Line Items]
Environmental	$ 16.6	$ 15.5
Total mine closure	225.4	211.4
Total environmental and mine closure obligations	242.0	226.9
Less current portion	15.7	13.7
Long term environmental and mine closure obligations	226.3	213.2
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member]
Loss Contingencies [Line Items]
Total mine closure	17.1	16.5
Owned Or Operating Facilities [Member] | U.S. Iron Ore [Member]
Loss Contingencies [Line Items]
Total mine closure	79.7	74.3
Owned Or Operating Facilities [Member] | Eastern Canadian Iron Ore [Member]
Loss Contingencies [Line Items]
Total mine closure	73.2	68.0
Owned Or Operating Facilities [Member] | Asia Pacific Iron Ore [Member]
Loss Contingencies [Line Items]
Total mine closure	17.3	16.3
Owned Or Operating Facilities [Member] | North American Coal [Member]
Loss Contingencies [Line Items]
Total mine closure	$ 38.1	$ 36.3