Quarterly report pursuant to Section 13 or 15(d)

Inventories (Schedule Of Inventories) (Detail)

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Inventories (Schedule Of Inventories) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Inventories [Line Items]    
Finished Goods $ 339.8 $ 273.3
Work-in Process 186.9 183.6
Total Inventory 526.7 456.9
U.S. Iron Ore [Member]
   
Inventories [Line Items]    
Finished Goods 182.7 100.2
Work-in Process 46.8 8.5
Total Inventory 229.5 108.7
Eastern Canadian Iron Ore [Member]
   
Inventories [Line Items]    
Finished Goods 77.7 96.2
Work-in Process 37.6 43.0
Total Inventory 115.3 139.2
Asia Pacific Iron Ore [Member]
   
Inventories [Line Items]    
Finished Goods 21.7 57.2
Work-in Process 41.8 21.6
Total Inventory 63.5 78.8
North American Coal [Member]
   
Inventories [Line Items]    
Finished Goods 57.7 19.7
Work-in Process 60.7 110.5
Total Inventory $ 118.4 $ 130.2