Statements Of Condensed Consolidated Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ 174.1	$ (749.3)	$ (7,224.2)
OTHER COMPREHENSIVE INCOME (LOSS)
Pension and OPEB liability, net of tax	(19.8)	45.2	(91.0)
Unrealized net gain (loss) on marketable securities, net of tax	0.0	1.7	(7.2)
Unrealized net gain (loss) on foreign currency translation	18.6	155.6	(42.3)
Unrealized net gain (loss) on derivative financial instruments, net of tax	(2.6)	20.7	2.8
OTHER COMPREHENSIVE INCOME (LOSS)	(3.8)	223.2	(137.7)
OTHER COMPREHENSIVE LOSS ATTRIBUTABLE TO THE NONCONTROLLING INTEREST	(0.5)	(4.6)	(4.8)
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ 170.8	$ (521.5)	$ (7,357.1)