QUARTERLY RESULTS OF OPERATIONS (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues	$ 476.0	$ 593.2	$ 498.1	$ 446.0	$ 1,030.3	$ 979.7	$ 747.7	$ 615.5	$ 2,013.3	$ 3,373.2	$ 3,890.8
SALES MARGIN	43.3	55.1	57.3	80.8	256.0	256.2	183.5	190.0	236.5	885.7	1,484.4
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	(34.8)	49.9	(38.2)	166.8	170.0	(274.2)	90.9	69.7	143.7	56.4	878.9
Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest	(2.4)	4.6	(5.0)	1.9	(3.4)	(2.5)	(3.6)	0.4	(8.6)	(25.9)	(14.8)
Income (Loss) from Continuing Operations Attributable to Parent	(37.2)	54.5	(43.2)	168.7	166.6	(276.7)	87.3	70.1	135.1	30.5	864.1
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(23.1)	(43.9)	103.4	(928.5)	(1,451.8)	(5,602.9)	(76.4)	(140.4)	(884.4)	(7,254.7)	(450.6)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	(60.3)	10.6	60.2	(759.8)	(1,285.2)	(5,879.6)	10.9	(70.3)	(749.3)	(7,224.2)	413.5
Dividends, Preferred Stock									38.4	51.2	48.7
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS COMMON SHAREHOLDERS	$ (60.3)	$ (15.0)	$ 60.2	$ (772.6)	$ (1,298.0)	$ (5,892.4)	$ (1.9)	$ (83.1)	$ (787.7)	$ (7,275.4)	$ 364.8
Continuing operations	$ (0.24)	$ 0.19	$ (0.28)	$ 1.02	$ 1.00	$ (1.89)	$ 0.49	$ 0.37	$ 0.63	$ (0.14)	$ 5.37
Discontinued operations	(0.15)	(0.29)	0.67	(6.06)	(9.48)	(36.60)	(0.50)	(0.92)	(5.77)	(47.38)	(2.97)
Earnings Per Share, Basic	(0.39)	(0.10)	0.39	(5.04)	(8.48)	(38.49)	(0.01)	(0.55)	(5.14)	(47.52)	2.40
Continuing operations	(0.24)	0.19	(0.28)	0.94	0.94	(1.89)	0.48	0.37	0.63	(0.14)	4.95
Discontinued operations	(0.15)	(0.29)	0.67	(5.20)	(8.13)	(36.60)	(0.50)	(0.91)	(5.76)	(47.38)	(2.58)
Earnings Per Share, Diluted	$ (0.39)	$ (0.10)	$ 0.39	$ (4.26)	$ (7.19)	$ (38.49)	$ (0.02)	$ (0.54)	$ (5.13)	$ (47.52)	$ 2.37
Preferred Class A [Member]
Dividends, Preferred Stock	$ 0.0	$ 25.6	$ 0.0		$ 12.8	$ 12.8	$ 12.8	$ 12.8	$ 38.4	$ 51.2	$ 48.7