Quarterly report pursuant to Section 13 or 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
Mar. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Segment Reporting Information [Line Items]      
Assets $ 2,862.9 $ 2,990.2 $ 2,953.4
U.S. Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 1,646.8   1,500.6
Asia Pacific Iron Ore [Member]      
Segment Reporting Information [Line Items]      
Assets 78.4   138.8
Total Segment Assets [Member]      
Segment Reporting Information [Line Items]      
Assets 1,725.2   1,639.4
Corporate [Member]      
Segment Reporting Information [Line Items]      
Assets $ 1,137.7   $ 1,314.0