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

SEGMENT REPORTING (Narrative) (Details) $ in Millions	3 Months Ended								12 Months Ended
SEGMENT REPORTING (Narrative) (Details) $ in Millions	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) customer	Dec. 31, 2016 USD ($) customer	Dec. 31, 2015 USD ($) customer
Segment Reporting Information [Line Items]
Revenues	$ 600.9	$ 698.4	$ 569.3	$ 461.6	$ 754.0	$ 553.3	$ 496.2	$ 305.5	$ 2,330.2	$ 2,109.0	$ 2,013.3
Sales Revenue, Net [Member] \| Customer Concentration Risk [Member]
Segment Reporting Information [Line Items]
Number of Customers \| customer									2	2	3
Concentration Risk, Percentage									10.00%	10.00%	10.00%
Revenues									$ 1,300.0	$ 1,100.0	$ 1,300.0