Quarterly report pursuant to Section 13 or 15(d)

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

v2.4.0.8
SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Segment Reporting Information [Line Items]    
Assets $ 13,102.4 $ 13,121.9
U.S. Iron Ore [Member]
   
Segment Reporting Information [Line Items]    
Assets 1,825.2 1,671.6
Eastern Canadian Iron Ore [Member]
   
Segment Reporting Information [Line Items]    
Assets 7,740.8 7,915.5
Asia Pacific Iron Ore [Member]
   
Segment Reporting Information [Line Items]    
Assets 1,046.0 1,078.4
North American Coal [Member]
   
Segment Reporting Information [Line Items]    
Assets 1,750.6 1,841.8
All Other Segments [Member]
   
Segment Reporting Information [Line Items]    
Assets 513.4 455.6
Total Segment Assets [Member]
   
Segment Reporting Information [Line Items]    
Assets 12,876.0 12,962.9
Corporate [Member]
   
Segment Reporting Information [Line Items]    
Assets $ 226.4 $ 159.0