SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Assets	$ 2,030.1	$ 1,923.9
U.S. Iron Ore [Member]
Segment Reporting Information [Line Items]
Assets	1,525.9	1,372.5
Asia Pacific Iron Ore [Member]
Segment Reporting Information [Line Items]
Assets	157.7	155.1
Total Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	1,683.6	1,527.6
Corporate [Member]
Segment Reporting Information [Line Items]
Assets	$ 346.5	$ 396.3