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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount			$ 1,106.4
Gain (loss) on extinguishment/restructuring of debt	$ (4.9)	$ 3.6	(76.8)	$ 182.4
$540 Million 8.25% 2020 Lien Notes [Member]
Extinguishment of Debt [Line Items]
Extinguishment of Debt, Amount			35.6
Gain (loss) on extinguishment/restructuring of debt	(4.9)		(4.9)
$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			136.8
Gain (loss) on extinguishment/restructuring of debt	0.0		(7.8)
Five Hundred Million Four Point Eight Zero Percent Senior Note [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)