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

FAIR VALUE OF FINANCIAL INSTRUMENTS (Fair Value Of Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets:
Cash equivalents	$ 616.9	$ 177.0
Derivative Asset	39.4	33.1
Total Asset	656.3	210.1
Liabilities:
Derivative Liability	2.7	0.5
Total Liability	2.7	0.5
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	66.3	177.0
Derivative Asset	0.0	0.0
Total Asset	66.3	177.0
Liabilities:
Derivative Liability	0.0	0.0
Total Liability	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash equivalents	550.6	0.0
Derivative Asset	0.0	1.5
Total Asset	550.6	1.5
Liabilities:
Derivative Liability	0.3	0.0
Total Liability	0.3	0.0
Fair Value, Inputs, Level 3 [Member]
Assets:
Cash equivalents	0.0	0.0
Derivative Asset	39.4	31.6
Total Asset	39.4	31.6
Liabilities:
Derivative Liability	2.4	0.5
Total Liability	$ 2.4	$ 0.5