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

DEBT AND CREDIT FACILITIES DEBT AND CREDIT FACILITIES (Schedule of Extinguishment of Debt) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount	$ 1.0	$ 724.0	$ 16.5
Gain (loss) on extinguishment of debt	0.0	(18.2)	0.2
$400 million 5.90% 2020 Senior Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount	0.0		0.5
Gain (loss) on extinguishment of debt	0.0		0.0
$500 Million 4.80% 2020 Senior Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount	0.0		0.1
Gain (loss) on extinguishment of debt	0.0		0.0
$700 Million 4.875% 2021 Senior Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount	1.0	124.0	14.2
Gain (loss) on extinguishment of debt	0.0	(5.3)	0.1
$1.075 Billion 5.75% 2025 Senior Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount	0.0	600.0	1.7
Gain (loss) on extinguishment of debt	$ 0.0	$ (12.9)	$ 0.1