EARNINGS PER SHARE (Earnings Per Share Computation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount											25,300	25,200
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	$ 100.1	$ (25.1)	$ 29.9	$ 114.3	$ (34.8)	$ 49.9	$ (38.2)	$ 166.8		$ 219.2	$ 143.7	$ 56.4
Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest	(1.7)	2.0	(16.7)	(8.8)	(2.4)	4.6	(5.0)	1.9		(25.2)	(8.6)	(25.9)
Income (Loss) from Continuing Operations Attributable to Parent	98.4	(23.1)	13.2	105.5		54.5	(43.2)	168.7	$ (37.2)	194.0	135.1	30.5
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(19.3)	(2.7)	(0.4)	2.5	$ (23.1)	(43.9)	103.4	(928.5)		(19.9)	(884.4)	(7,254.7)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS						10.6	60.2	(759.8)	(60.3)	174.1	(749.3)	(7,224.2)
PREFERRED STOCK DIVIDENDS										0.0	(38.4)	(51.2)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS COMMON SHAREHOLDERS	$ 79.1	$ (25.8)	$ 12.8	$ 108.0		$ (15.0)	$ 60.2	$ (772.6)	$ (60.3)	$ 174.1	$ (787.7)	$ (7,275.4)
Weighted Average Number of Shares:
Basic										197,659	153,230	153,098
Employee Stock Plans										2,386	375	0
Diluted										200,145	153,605	153,098
Earnings (loss) per Common Share Attributable to Cliffs Common Shareholders - Basic:
Continuing operations	$ 0.43	$ (0.11)	$ 0.07	$ 0.61	$ (0.24)	$ 0.19	$ (0.28)	$ 1.02		$ 0.98	$ 0.63	$ (0.14)
Discontinued operations	(0.08)	(0.01)	0.00	0.01	(0.15)	(0.29)	0.67	(6.06)		(0.10)	(5.77)	(47.38)
Earnings (loss) per Common Share Attributable to Cliffs Common Shareholders - Basic:	0.35	(0.12)	0.07	0.62	(0.39)	(0.10)	0.39	(5.04)		0.88	(5.14)	(47.52)
Earnings (loss) per Common Share Attributable to Cliffs Common Shareholders - Diluted:
Continuing operations	0.42	(0.11)	0.07	0.61	(0.24)	0.19	(0.28)	0.94		0.97	0.63	(0.14)
Discontinued operations	(0.08)	(0.01)	0.00	0.01	(0.15)	(0.29)	0.67	(5.20)		(0.10)	(5.76)	(47.38)
Earnings (loss) per Common Share Attributable to Cliffs Common Shareholders - Diluted:	$ 0.34	$ (0.12)	$ 0.07	$ 0.62	$ (0.39)	$ (0.10)	$ 0.39	$ (4.26)		$ 0.87	$ (5.13)	$ (47.52)
Preferred Class A [Member]
PREFERRED STOCK DIVIDENDS					$ 0.0	$ (25.6)	$ 0.0	$ (12.8)		$ 0.0	$ (38.4)	$ (51.2)