Annual report pursuant to Section 13 and 15(d)

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

v3.3.1.900
SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Segment Reporting Information [Line Items]      
Assets $ 2,135.5 $ 3,147.2 $ 13,102.9
U.S. Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 1,476.4 1,464.9 1,537.9
Asia Pacific Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 202.5 306.2 1,176.8
Total Segment Assets [Member]      
Segment Reporting Information [Line Items]      
Assets 1,678.9 1,771.1 2,714.7
Corporate [Member]      
Segment Reporting Information [Line Items]      
Assets 441.7 666.2 204.2
Discontinued Operations [Member]      
Segment Reporting Information [Line Items]      
Disposal Group, Including Discontinued Operation, Assets $ 14.9 $ 709.9 $ 10,184.0