Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
SEGMENT REPORTING SEGMENT REPORTING (Summary of Assets by Segment) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
Segment Reporting Information [Line Items]      
Assets $ 3,051.5 $ 2,990.2 $ 2,953.4
U.S. Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 1,822.8   1,500.6
Corporate Segment [Member]      
Segment Reporting Information [Line Items]      
Assets 1,183.4   1,314.0
Discontinued Operations, Held-for-sale [Member]      
Segment Reporting Information [Line Items]      
Assets $ 45.3   $ 138.8