Quarterly report pursuant to Section 13 or 15(d)

GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Schedule Of Goodwill) (Details)

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GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Schedule Of Goodwill) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Goodwill [Roll Forward]    
Beginning Balance $ 167.4 $ 1,152.1
Arising in business combinations 0 13.8
Impairment 0 (1,000.0)
Impact of foreign currency translation 0.2 1.5
Ending Balance 167.6 167.4
Accumulated Goodwill Impairment Loss (1,027.8) (1,027.8)
U.S. Iron Ore [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 2.0 2.0
Arising in business combinations 0 0
Impairment 0 0
Impact of foreign currency translation 0 0
Ending Balance 2.0 2.0
Accumulated Goodwill Impairment Loss 0 0
Eastern Canadian Iron Ore [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 0 986.2
Arising in business combinations 0 13.8
Impairment 0 (1,000.0)
Impact of foreign currency translation 0 0
Ending Balance 0 0
Accumulated Goodwill Impairment Loss (1,000.0) (1,000.0)
Asia Pacific Iron Ore [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 84.5 83.0
Arising in business combinations 0 0
Impairment 0 0
Impact of foreign currency translation 0.2 1.5
Ending Balance 84.7 84.5
Accumulated Goodwill Impairment Loss 0 0
North American Coal [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 0 0
Arising in business combinations 0 0
Impairment 0 0
Impact of foreign currency translation 0 0
Ending Balance 0 0
Accumulated Goodwill Impairment Loss (27.8) (27.8)
Other Segment [Member]
    
Goodwill [Roll Forward]    
Beginning Balance 80.9 80.9
Arising in business combinations 0 0
Impairment 0 0
Impact of foreign currency translation 0 0
Ending Balance 80.9 80.9
Accumulated Goodwill Impairment Loss $ 0 $ 0