Quarterly report pursuant to Section 13 or 15(d)

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

v3.7.0.1
SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Segment Reporting Information [Line Items]    
Assets $ 1,925.7 $ 1,923.9
U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Assets 1,440.6 1,372.5
Asia Pacific Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Assets 168.4 155.1
Total Segment Assets [Member]    
Segment Reporting Information [Line Items]    
Assets 1,609.0 1,527.6
Corporate [Member]    
Segment Reporting Information [Line Items]    
Assets $ 316.7 $ 396.3