Annual report pursuant to Section 13 and 15(d)

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

v3.8.0.1
SEGMENT REPORTING (Summary of Assets by Segment) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]      
Assets $ 2,953.4 $ 1,923.9 $ 2,135.5
U.S. Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 1,500.6 1,372.5 1,476.4
Asia Pacific Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 138.8 155.1 202.5
Total Segment Assets [Member]      
Segment Reporting Information [Line Items]      
Assets 1,639.4 1,527.6 1,678.9
Corporate [Member]      
Segment Reporting Information [Line Items]      
Assets 1,314.0 396.3 441.7
Discontinued Operations [Member]      
Segment Reporting Information [Line Items]      
Disposal Group, Including Discontinued Operation, Assets $ 0.0 $ 0.0 $ 14.9