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

FAIR VALUE MEASUREMENTS (Fair Value of Assets and Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	$ 526.6		$ 616.9
Derivative assets	93.6	$ 51.0	39.4
Total Asset	620.2		656.3
Derivative Liability	4.4		2.7
Total Liability	4.4		2.7
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	36.0		66.3
Derivative assets	0.0		0.0
Total Asset	36.0		66.3
Derivative Liability	0.0		0.0
Total Liability	0.0		0.0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	490.6		550.6
Derivative assets	0.0		0.0
Total Asset	490.6		550.6
Derivative Liability	0.2		0.3
Total Liability	0.2		0.3
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	0.0		0.0
Derivative assets	93.6		39.4
Total Asset	93.6		39.4
Derivative Liability	4.2		2.4
Total Liability	$ 4.2		$ 2.4