Statements Of Condensed Consolidated Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
NET LOSS ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ (84.3)	$ (28.1)
OTHER COMPREHENSIVE INCOME (LOSS)
Changes in pension and other post-retirement benefits, net of tax	6.7	4.7
Unrealized net gain (loss) on foreign currency translation	0.7	(12.7)
Unrealized net gain on derivative financial instruments, net of tax	0.3	0.0
OTHER COMPREHENSIVE INCOME (LOSS)	7.7	(8.0)
OTHER COMPREHENSIVE LOSS ATTRIBUTABLE TO THE NONCONTROLLING INTEREST	0.0	(5.0)
TOTAL COMPREHENSIVE LOSS ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ (76.6)	$ (31.1)