Statements Of Condensed Consolidated Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ 53.4	$ (25.8)	$ 57.1	$ 95.0
OTHER COMPREHENSIVE INCOME (LOSS)
Changes in pension and other post-retirement benefits, net of tax	7.5	7.1	18.9	19.0
Unrealized net gain (loss) on foreign currency translation	0.5	0.9	(13.6)	2.6
Unrealized net gain (loss) on derivative financial instruments, net of tax	0.0	0.7	0.0	(2.6)
OTHER COMPREHENSIVE INCOME	8.0	8.7	5.3	19.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	5.7	0.9	1.1	2.2
Other comprehensive income (loss)	$ 55.7	$ (18.0)	$ 61.3	$ 111.8