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
 9 Months Ended 12 Months Ended
Sep. 30, 2011
Dec. 31, 2010
Goodwill, beginning balance $ 196.5 [1] $ 74.6 [1]
Arising in business combinations 1,045.8 111.9 [1]
Impact of foreign currency translation (4.1) 10.0 [1]
Other (0.5)  
Goodwill, ending balance 1,237.7 196.5 [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 [1]
Eastern Canadian Iron Ore [Member]
    
Goodwill, beginning balance 3.1 [1]  
Arising in business combinations 1,045.8 3.1 [1]
Other (0.4)  
Goodwill, ending balance 1,048.5 3.1 [1]
Asia Pacific Iron Ore [Member]
    
Goodwill, beginning balance 82.6 [1] 72.6 [1]
Impact of foreign currency translation (4.1) 10.0 [1]
Goodwill, ending balance 78.5 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 27.9 [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 [1]
[1] Represents a 12-month rollforward of our goodwill by reportable unit at December 31, 2010.