Annual report pursuant to Section 13 and 15(d)

INVENTORIES (Schedule Of Inventories) (Details)

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INVENTORIES (Schedule Of Inventories) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Inventory, Net [Abstract]      
Finished Goods $ 283.2 $ 273.3  
Work-in Process 153.3 183.6  
Total Inventory 436.5 456.9  
U.S. Iron Ore [Member]
      
Inventory, Net [Abstract]      
Finished Goods 147.2 100.2  
Work-in Process 22.9 8.5  
Total Inventory 170.1 108.7  
Excess of current cost over LIFO cost of iron ore inventories 122.2 117.1  
Increase in Inventories due to build up of LIFO layers 47.5    
Eastern Canadian Iron Ore [Member]
      
Inventory, Net [Abstract]      
Finished Goods 62.6 96.2  
Work-in Process 44.2 43.0  
Total Inventory 106.8 139.2  
Excess of current cost over LIFO cost of iron ore inventories 27.7 21.9  
Increase in Inventories due to build up of LIFO layers   47.1  
Asia Pacific Iron Ore [Member]
      
Inventory, Net [Abstract]      
Finished Goods 36.7 57.2  
Work-in Process 37.2 21.6  
Total Inventory 73.9 78.8  
North American Coal [Member]
      
Inventory, Net [Abstract]      
Finished Goods 36.7 19.7  
Work-in Process 49.0 110.5  
Total Inventory 85.7 130.2  
Lower-of-cost-or-market inventory charges 24.4 6.6 26.1
Cost Of Goods Sold And Operating Expenses [Member] | U.S. Iron Ore [Member]
      
Inventory, Net [Abstract]      
Effect of LIFO Inventory Liquidation on Income   (15.2)  
Cost Of Goods Sold And Operating Expenses [Member] | Eastern Canadian Iron Ore [Member]
      
Inventory, Net [Abstract]      
Effect of LIFO Inventory Liquidation on Income $ (7.0)