Statements Of Unaudited Condensed Consolidated Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (108.1)	$ 160.8	$ (156.7)	$ 138.7
Other comprehensive income (loss):
Changes in pension and OPEB, net of tax	6.0	5.8	11.6	11.5
Changes in foreign currency translation	0.7	0.0	(0.2)	0.0
Changes in derivative financial instruments, net of tax	4.5	(2.1)	1.5	0.6
Total other comprehensive income	11.2	3.7	12.9	12.1
Comprehensive income (loss)	(96.9)	164.5	(143.8)	150.8
Comprehensive income attributable to noncontrolling interests	(15.8)	0.0	(19.3)	0.0
Comprehensive income (loss) attributable to Cliffs shareholders	$ (112.7)	$ 164.5	$ (163.1)	$ 150.8