Quarterly report pursuant to Section 13 or 15(d)

Goodwill And Other Intangible Assets And Liabilities (Schedule Of Goodwill) (Detail)

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Goodwill And Other Intangible Assets And Liabilities (Schedule Of Goodwill) (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Goodwill [Roll Forward]    
Beginning Balance $ 1,152.1 $ 196.5
Arising in business combinations 13.8 983.4
Impairment    (27.8)
Impact of foreign currency translation 1.3 0.4
Other    (0.4)
Ending Balance 1,167.2 1,152.1
U.S. Iron Ore [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 2.0 [1] 2.0 [1]
Arising in business combinations       [1]
Impairment       [1]
Impact of foreign currency translation       [1]
Other       [1]
Ending Balance 2.0 2.0 [1]
Eastern Canadian Iron Ore [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 986.2 [1] 3.1 [1]
Arising in business combinations 13.8 983.5 [1]
Impairment       [1]
Impact of foreign currency translation       [1]
Other    (0.4) [1]
Ending Balance 1,000.0 986.2 [1]
Asia Pacific Iron Ore [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 83.0 [1] 82.6 [1]
Arising in business combinations       [1]
Impairment       [1]
Impact of foreign currency translation 1.3 0.4 [1]
Other       [1]
Ending Balance 84.3 83.0 [1]
North American Coal [Member]
    
Goodwill [Roll Forward]    
Beginning Balance   27.9 [1]
Arising in business combinations   (0.1) [1]
Impairment   (27.8) [1]
Impact of foreign currency translation      [1]
Other      [1]
Ending Balance     
Other Segment [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 80.9 [1] 80.9 [1]
Arising in business combinations       [1]
Impairment       [1]
Impact of foreign currency translation       [1]
Other       [1]
Ending Balance $ 80.9 $ 80.9 [1]
[1] Represents a 12-Month rollforward of our goodwill by reportable segment at December 31, 2011.