SHAREHOLDERS' EQUITY Schedule of Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity Attributable to Parent	$ (833.3)		$ (833.3)		$ (1,464.3)
Stockholders' Equity Attributable to Noncontrolling Interest	0.2		0.2		133.8
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(833.1)	$ (1,400.5)	(833.1)	$ (1,400.5)	(1,330.5)	$ (1,811.6)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	53.4	(25.8)	57.1	95.0
Net Income (Loss) Attributable to Noncontrolling Interest	(0.5)	(2.0)	(3.9)	23.5
Net Income (Loss)	52.9	(27.8)	53.2	118.5
Other comprehensive income (loss)	55.7	(18.0)	61.3	111.8
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	5.7	0.9	1.1	2.2
Other Comprehensive Income (Loss), Net of Tax	8.0	8.7	5.3	19.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest			58.5	137.5
Stock and Other Incentive Plans			13.5	10.1
Distributions to partnership equity				(48.8)
Distributions to noncontrolling interest			1.8	(2.9)
Cliffs Shareholders Equity [Member]
Stockholders' Equity Attributable to Parent	(833.3)	(1,544.3)	(833.3)	(1,544.3)	(1,464.3)	(1,981.4)
Net Income (Loss) Available to Common Stockholders, Basic			57.1	95.0
Other comprehensive income (loss)			4.2	16.8
TOTAL COMPREHENSIVE INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS			61.3	111.8
Stock and Other Incentive Plans			13.5	10.1
Distributions to partnership equity				0.0
Distributions to noncontrolling interest			0.0	0.0
Noncontrolling Interest [Member]
Stockholders' Equity Attributable to Noncontrolling Interest	0.2	$ 143.8	0.2	143.8	$ 133.8	$ 169.8
Net Income (Loss) Attributable to Noncontrolling Interest			(3.9)	23.5
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest			1.1	2.2
OTHER COMPREHENSIVE LOSS (INCOME) ATTRIBUTABLE TO THE NONCONTROLLING INTEREST			(2.8)	25.7
Stock and Other Incentive Plans			0.0	0.0
Distributions to partnership equity				(48.8)
Distributions to noncontrolling interest			1.8	(2.9)
Common Stock [Member]
Stock Issued During Period, Value, New Issues			661.3	315.2
Common Stock [Member] | Cliffs Shareholders Equity [Member]
Stock Issued During Period, Value, New Issues			661.3	315.2
Common Stock [Member] | Noncontrolling Interest [Member]
Stock Issued During Period, Value, New Issues			0.0	$ 0.0
Tilden [Member]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	15.9		(105.0)
Tilden [Member] | Cliffs Shareholders Equity [Member]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests			(89.1)
Tilden [Member] | Noncontrolling Interest [Member]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests			(15.9)
Empire [Member]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	$ 116.7		(132.7)
Empire [Member] | Cliffs Shareholders Equity [Member]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests			(16.0)
Empire [Member] | Noncontrolling Interest [Member]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests			$ (116.7)