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

DEBT AND CREDIT FACILITIES DEBT AND CREDIT FACILITIES (Debt Restructuring) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				$ 1,610.7
Gain (loss) on extinguishment/restructuring of debt	$ (88.6)		$ (18.3)	(165.4)	$ 164.1
$540 Million 8.25% 2020 Lien Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount		$ 35.6		540.0
Gain (loss) on extinguishment/restructuring of debt	(88.6)			(93.5)
$218.5 Million 8.00% 2020 Lien Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				218.5
Gain (loss) on extinguishment/restructuring of debt	0.0			45.1
$544 Million 7.75% 2020 Lien Notes[Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				430.1
Gain (loss) on extinguishment/restructuring of debt	0.0			(104.5)
$400 Million 5.90% 2020 Senior Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				136.7
Gain (loss) on extinguishment/restructuring of debt	0.0			(7.8)
$500 million 4.80% 2020 Senior Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				114.4
Gain (loss) on extinguishment/restructuring of debt	0.0			(1.9)
$700 Million 4.875% 2021 Senior Note [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				171.0
Gain (loss) on extinguishment/restructuring of debt	$ 0.0			$ (2.8)