Cleveland-Cliffs Inc. Annual report pursuant to Section 13 and 15(d)

DEBT AND CREDIT FACILITIES (Debt Extinguishment) (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				$ 1,610.7
Gain (loss) on extinguishment/restructuring of debt			$ 174.3	(165.4)	$ 166.3	$ 392.9
$540 Million 8.25% 2020 First Lien Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount		$ 35.6		540.0
Gain (loss) on extinguishment/restructuring of debt				(93.5)
$218.5 Million 8.00% 2020 1.5 Lien Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				218.5
Gain (loss) on extinguishment/restructuring of debt				45.1
$544.2 Million 7.75% 2020 Second Lien Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				430.1
Gain (loss) on extinguishment/restructuring of debt				(104.5)	6.9
Four Hundred Million Five Point Nine Zero Two Thousand And Twenty Senior Note [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				136.7
Gain (loss) on extinguishment/restructuring of debt				(7.8)	28.3
$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	$ 0.6			(1.9)	19.5
$700 Million 4.875% 2021 Senior Notes[Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount				171.0
Gain (loss) on extinguishment/restructuring of debt				$ (2.8)	$ 33.3