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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] Ìý Ìý
Beginning Balance $ 1,152.1 $ 196.5
Arising in business combinations 13.8 983.4
Impairment ÌýÌý (27.8)
Impact of foreign currency translation 1.3 0.4
Other ÌýÌý (0.4)
Ending Balance 1,167.2 1,152.1
U.S. Iron Ore [Member]
 Ìý Ìý
Goodwill [Roll Forward] Ìý Ìý
Beginning Balance 2.0 [1] 2.0 [1]
Arising in business combinations ÌýÌý ÌýÌý [1]
Impairment ÌýÌý ÌýÌý [1]
Impact of foreign currency translation ÌýÌý ÌýÌý [1]
Other ÌýÌý ÌýÌý [1]
Ending Balance 2.0 2.0 [1]
Eastern Canadian Iron Ore [Member]
 Ìý Ìý
Goodwill [Roll Forward] Ìý Ìý
Beginning Balance 986.2 [1] 3.1 [1]
Arising in business combinations 13.8 983.5 [1]
Impairment ÌýÌý ÌýÌý [1]
Impact of foreign currency translation ÌýÌý ÌýÌý [1]
Other ÌýÌý (0.4) [1]
Ending Balance 1,000.0 986.2 [1]
Asia Pacific Iron Ore [Member]
 Ìý Ìý
Goodwill [Roll Forward] Ìý Ìý
Beginning Balance 83.0 [1] 82.6 [1]
Arising in business combinations ÌýÌý ÌýÌý [1]
Impairment ÌýÌý ÌýÌý [1]
Impact of foreign currency translation 1.3 0.4 [1]
Other ÌýÌý ÌýÌý [1]
Ending Balance 84.3 83.0 [1]
North American Coal [Member]
 Ìý Ìý
Goodwill [Roll Forward] Ìý Ìý
Beginning Balance Ìý 27.9 [1]
Arising in business combinations Ìý (0.1) [1]
Impairment Ìý (27.8) [1]
Impact of foreign currency translation Ìý ÌýÌý [1]
Other Ìý ÌýÌý [1]
Ending Balance Ìý ÌýÌý
Other Segment [Member]
 Ìý Ìý
Goodwill [Roll Forward] Ìý Ìý
Beginning Balance 80.9 [1] 80.9 [1]
Arising in business combinations ÌýÌý ÌýÌý [1]
Impairment ÌýÌý ÌýÌý [1]
Impact of foreign currency translation ÌýÌý ÌýÌý [1]
Other ÌýÌý ÌýÌý [1]
Ending Balance $ 80.9 $ 80.9 [1]
[1] Represents a 12-Month rollforward of our goodwill by reportable segment at DecemberÌý31, 2011.