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
 6 Months Ended 12 Months Ended 6 Months Ended 12 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2012
U.S. Iron Ore [Member]
Dec. 31, 2011
U.S. Iron Ore [Member]
Dec. 31, 2010
U.S. Iron Ore [Member]
Jun. 30, 2012
Eastern Canadian Iron Ore [Member]
Dec. 31, 2011
Eastern Canadian Iron Ore [Member]
Dec. 31, 2011
North American Coal [Member]
Jun. 30, 2012
Asia Pacific Iron Ore [Member]
Dec. 31, 2011
Asia Pacific Iron Ore [Member]
Jun. 30, 2012
Other [Member]
Dec. 31, 2011
Other [Member]
Dec. 31, 2010
Other [Member]
Region Reporting Information, by Region [Line Items]                          
Beginning Balance $ 1,152.1 [1] $ 196.5 [1] $ 2.0 $ 2.0 [1] $ 2.0 [1] $ 986.2 [1] $ 3.1 [1] $ 27.9 [1] $ 83.0 [1] $ 82.6 [1] $ 80.9 $ 80.9 [1] $ 80.9 [1]
Arising in business combinations 13.8 983.4 [1]       13.8 983.5 [1] (0.1) [1]          
Impairment   (27.8) [1]           (27.8) [1]          
Impact of foreign currency translation 0.2 0.4 [1]             0.2 0.4 [1]      
Other   (0.4) [1]         (0.4) [1]            
Ending Balance $ 1,166.1 $ 1,152.1 [1] $ 2.0 $ 2.0 [1] $ 2.0 [1] $ 1,000.0 $ 986.2 [1]   $ 83.2 $ 83.0 [1] $ 80.9 $ 80.9 [1] $ 80.9 [1]
[1] Represents a 12-Month rollforward of our goodwill by reportable unit at December 31, 2011.