Quarterly report pursuant to Section 13 or 15(d)

SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details)

v2.4.0.6
SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Segment Reporting Information [Line Items]    
Assets $ 13,821.9 $ 13,574.9
U.S. Iron Ore [Member]
   
Segment Reporting Information [Line Items]    
Assets 1,848.4 1,735.1
Eastern Canadian Iron Ore [Member]
   
Segment Reporting Information [Line Items]    
Assets 7,719.7 7,605.1
Asia Pacific Iron Ore [Member]
   
Segment Reporting Information [Line Items]    
Assets 1,474.4 1,506.3
North American Coal [Member]
   
Segment Reporting Information [Line Items]    
Assets 1,829.7 1,877.8
All Other Segments [Member]
   
Segment Reporting Information [Line Items]    
Assets 599.3 570.9
Total Segment Assets [Member]
   
Segment Reporting Information [Line Items]    
Assets 13,471.5 13,295.2
Corporate [Member]
   
Segment Reporting Information [Line Items]    
Assets $ 350.4 $ 279.7