Quarterly report pursuant to Section 13 or 15(d)

Goodwill and Other Intangible Assets and Liabilities (Changes in the Carrying Amount of Goodwill Allocated by Reporting Unit) (Details)

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Goodwill and Other Intangible Assets and Liabilities (Changes in the Carrying Amount of Goodwill Allocated by Reporting Unit) (Details) (USD $)
In Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2011
Jun. 30, 2011
Dec. 31, 2010
Goodwill, beginning balance   $ 196.5 [1] $ 74.6 [1]
Arising in business combinations 1,026.8 1,026.8 111.9 [1]
Impact of foreign currency translation   4.5 10.0 [1]
Other   (0.5)  
Goodwill, ending balance 1,227.3 1,227.3 196.5 [1]
U.S. Iron Ore [Member]
      
Goodwill, beginning balance   2.0 [1] 2.0 [1]
Arising in business combinations       [1]
Impact of foreign currency translation       [1]
Other       [1]
Goodwill, ending balance 2.0 2.0 2.0 [1]
Eastern Canadian Iron Ore [Member]
      
Goodwill, beginning balance   3.1 [1]  
Arising in business combinations   1,026.8 3.1 [1]
Other   (0.4)  
Goodwill, ending balance 1,029.5 1,029.5 3.1 [1]
Asia Pacific Iron Ore [Member]
      
Goodwill, beginning balance   82.6 [1] 72.6 [1]
Impact of foreign currency translation   4.5 10.0 [1]
Goodwill, ending balance 87.1 87.1 82.6 [1]
North American Coal [Member]
      
Goodwill, beginning balance   27.9 [1]  
Arising in business combinations     27.9 [1]
Other   (0.1)  
Goodwill, ending balance 27.8 27.8 27.9 [1]
Other Goodwill [Member]
      
Goodwill, beginning balance   80.9 [1]  
Arising in business combinations     80.9 [1]
Impact of foreign currency translation      
Other      
Goodwill, ending balance $ 80.9 $ 80.9 $ 80.9 [1]
[1] Represents a 12-month rollforward of our goodwill by reportable unit at December 31, 2010.