Annual report pursuant to Section 13 and 15(d)

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

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SEGMENT REPORTING (Summary of Assets by Segment) (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Segment Reporting Information [Line Items]      
Assets $ 3,503.8 $ 3,529.6 $ 2,953.4
Mining and Pelletizing [Member]      
Segment Reporting Information [Line Items]      
Assets 1,643.1 1,694.1 1,500.6
Metallics [Member]      
Segment Reporting Information [Line Items]      
Assets 913.6 265.9 13.4
Total Segment Assets [Member]      
Segment Reporting Information [Line Items]      
Assets 2,556.7 1,960.0 1,514.0
Corporate [Member]      
Segment Reporting Information [Line Items]      
Assets 940.1 1,557.2 1,300.6
Discontinued Operations [Member]      
Segment Reporting Information [Line Items]      
Assets of discontinued operations $ 7.0 $ 12.4 $ 138.8