EARNINGS PER SHARE (Earnings Per Share Computation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Loss from continuing operations	$ (49.2)	$ (22.1)
Income attributable to noncontrolling interest	(3.5)	0.0
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	(52.7)	(22.1)
Income from discontinued operations, net of tax	0.6	0.0
Net income (loss)	$ (52.1)	$ (22.1)
Weighted average number of shares:
Basic	297,515	289,525
Convertible Senior Notes	0	0
Employee stock plans	0	0
Diluted	297,515	289,525
Loss per common share attributable to Cliffs shareholders - basic:
Continuing operations (in dollars per share)	$ (0.18)	$ (0.08)
Discontinued operations (in dollars per share)	0	0
Earnings (Loss) per Common Share - Basic (in dollars per share)	(0.18)	(0.08)
Loss per common share attributable to Cliffs shareholders - diluted:
Continuing operations (in dollars per share)	(0.18)	(0.08)
Discontinued operations (in dollars per share)	0	0
Earnings (Loss) per Common Share - Diluted (in dollars per share)	$ (0.18)	$ (0.08)