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

CAPITAL STOCK (Details)

CAPITAL STOCK (Details) $ / shares in Units, $ in Millions	3 Months Ended				9 Months Ended	12 Months Ended
CAPITAL STOCK (Details) $ / shares in Units, $ in Millions	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) shares	Mar. 31, 2017 USD ($) shares	Mar. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Feb. 09, 2017 $ / shares	Aug. 10, 2016 $ / shares	Nov. 02, 2015 $ / shares	Aug. 03, 2015 $ / shares	May 01, 2015 $ / shares	Feb. 02, 2015 $ / shares
Class of Stock [Line Items]
Gain (loss) on extinguishment/restructuring of debt				$ 174.3		$ (165.4)	$ 166.3	$ 392.9
Common Stock, New Shares, Issued \| shares		44,400,000	63,250,000
Common shares, par value \| $ / shares	$ 0.125					$ 0.125	$ 0.125
Trading Day Window Determining Number of Common Shares Issuable on Conversion						20
Dividends, Preferred Stock						$ 0.0	$ 0.0	38.4
Common shares, issued (in shares) \| shares	301,886,794					301,886,794	238,636,794
Net proceeds from issuance of common shares					$ 287.6	$ 661.3	$ 287.4	0.0
Current Fiscal Year End Date						--12-31
Common Shares Issued in Debt to Equity Exchange \| shares	8,200,000					8,200,000
Debt Instrument, Face Amount Exchanged							$ 512.2
Shares Issued, Price Per Share \| $ / shares									$ 10.75	$ 6.75
Extinguishment of Debt, Amount						$ 1,610.7
Preferred Class A [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share \| $ / shares	$ 1,000					$ 1,000	$ 1,000
Dividends payable, per share \| $ / shares											$ 17.5000	$ 17.5000	$ 17.5000	$ 17.5000
Dividends, Preferred Stock						$ 0.0	$ 0.0	$ 38.4
Common Stock [Member]
Class of Stock [Line Items]
Convertible Preferred Stock, Common Shares Issued Upon Conversion in Lieu of Dividend \| shares	1,300,000					1,300,000
Convertible Preferred Stock, Shares Issued upon Conversion \| shares	0.7037					0.7037
Convertible Preferred Stock, Shares Issued Upon Conversion per Depositary Share \| shares	0.8621					0.8621
Common shares, issued (in shares) \| shares	26,500,000.0000					26,500,000.0000
Convertible Preferred Stock, Common Shares Issued Upon Conversion \| shares	25,200,000.0000					25,200,000.0000
Common Stock [Member] \| Minimum [Member]
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion \| shares	28.1480					28.1480
Convertible Preferred Stock, Shares Issued Upon Conversion per Depositary Share \| shares	0.9052					0.9052
Common Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Convertible Preferred Stock, Shares Issued upon Conversion \| shares	34.4840					34.4840
Depositary Shares [Member]
Class of Stock [Line Items]
Dividends payable, per share \| $ / shares											$ 0.44	$ 0.44	$ 0.44	$ 0.44
Exchange of Debt for Equity [Member]
Class of Stock [Line Items]
Gain (loss) on extinguishment/restructuring of debt	$ 11.3
$500 Million 3.95% 2018 Senior Notes [Member]
Class of Stock [Line Items]
Gain (loss) on extinguishment/restructuring of debt	19.9						1.8
Debt Instrument, Face Amount Exchanged							17.6
$500 Million 3.95% 2018 Senior Notes [Member] \| Exchange of Debt for Equity [Member]
Class of Stock [Line Items]
Debt Instrument, Face Amount Exchanged	10.0					$ 10.0
$218.5 Million 8.00% 2020 1.5 Lien Notes [Member]
Class of Stock [Line Items]
Gain (loss) on extinguishment/restructuring of debt						45.1
Extinguishment of Debt, Amount						218.5
$218.5 Million 8.00% 2020 1.5 Lien Notes [Member] \| Exchange of Debt for Equity [Member]
Class of Stock [Line Items]
Debt Instrument, Face Amount Exchanged	20.1					20.1
$700 Million 4.875% 2021 Senior Notes[Member]
Class of Stock [Line Items]
Gain (loss) on extinguishment/restructuring of debt						(2.8)	33.3
Debt Instrument, Face Amount Exchanged							$ 76.3
Extinguishment of Debt, Amount						171.0
$700 Million 4.875% 2021 Senior Notes[Member] \| Exchange of Debt for Equity [Member]
Class of Stock [Line Items]
Debt Instrument, Face Amount Exchanged	$ 26.8					26.8
$1.075 Billion 5.75% 2025 Senior Notes [Member]
Class of Stock [Line Items]
Proceeds from Issuance of Debt		$ 575.0	$ 500.0
Secured Debt [Member]
Class of Stock [Line Items]
Extinguishment of Debt, Amount						648.6
Unsecured Debt [Member]
Class of Stock [Line Items]
Extinguishment of Debt, Amount						422.2
$540 Million 8.25% 2020 First Lien Notes [Member]
Class of Stock [Line Items]
Gain (loss) on extinguishment/restructuring of debt						(93.5)
Extinguishment of Debt, Amount			$ 35.6			$ 540.0