Quarterly report pursuant to Section 13 or 15(d)

Segment Reporting (Schedule of Summary of Assets, by Segment) (Details)

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Segment Reporting (Schedule of Summary of Assets, by Segment) (Details) (USD $)
In Millions
Jun. 30, 2011
Dec. 31, 2010
Total assets $ 13,671.7 $ 7,778.2
U.S. Iron Ore [Member]
   
Total assets 1,807.6 1,537.1
Eastern Canadian Iron Ore [Member]
   
Total assets 7,525.6 629.6
North American Coal [Member]
   
Total assets 1,625.4 1,623.8
Asia Pacific Iron Ore [Member]
   
Total assets 1,429.2 1,195.3
Other Segment [Member]
   
Total assets 1,021.3 1,257.8
Total Segment [Member]
   
Total assets 13,409.1 6,243.6
Corporate [Member]
   
Total assets $ 262.6 $ 1,534.6