Cleveland-Cliffs Inc. Quarterly report pursuant to Section 13 or 15(d)

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details)

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 261.7	$ 424.2			$ 424.2
Balance, beginning of period PY			$ (484.8)	$ (444.1)		$ (444.1)
Balance, end of period	285.6	261.7			285.6
Balance, end of period PY			(306.3)	(484.8)		(306.3)
Changes in Pension and Other Post-Retirement Benefits, net of tax [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(275.4)	(281.1)			(281.1)
Balance, beginning of period PY			(257.2)	(263.9)		(263.9)
Other comprehensive income (loss) before reclassifications	0.3	0.2	0.2	0.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	5.5	5.5	6.5	6.2
Balance, end of period	(269.6)	(275.4)			(269.6)
Balance, end of period PY			(250.5)	(257.2)		(250.5)
Unrealized Net Gain (Loss) on Foreign Currency Translation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period PY			226.1	225.4		225.4
Other comprehensive income (loss) before reclassifications			2.2	0.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			0.0	0.0
Balance, end of period PY			228.3	226.1		228.3
Net Unrealized Gain (Loss) on Derivative Financial Instruments, net of tax [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(0.1)	(2.8)			(2.8)
Balance, beginning of period PY			(0.2)	(0.5)		(0.5)
Other comprehensive income (loss) before reclassifications	(2.3)	2.5	0.2	0.4
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	0.2	0.0	(0.1)
Balance, end of period	(2.2)	(0.1)			(2.2)
Balance, end of period PY			0.0	(0.2)		0.0
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(275.5)	(283.9)			(283.9)
Balance, beginning of period PY			(31.3)	(39.0)		(39.0)
Other comprehensive income (loss) before reclassifications	(2.0)	2.7	2.6	1.6
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	5.7	5.7	6.5	6.1
Balance, end of period	$ (271.8)	$ (275.5)			$ (271.8)
Balance, end of period PY			$ (22.2)	$ (31.3)		$ (22.2)