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

SEGMENTS REPORTING (Revenue by Geographical Location) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 696.3	$ 741.8	$ 714.3	$ 180.0	$ 511.8	$ 596.7	$ 471.3	$ 286.2	$ 2,332.4	$ 1,866.0	$ 1,554.5
PROPERTY, PLANT AND EQUIPMENT, NET	1,286.0				1,033.8				1,286.0	1,033.8		$ 1,033.8
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									1,847.3	1,504.5	1,236.2
PROPERTY, PLANT AND EQUIPMENT, NET	$ 1,286.0				$ 1,033.8				1,286.0	1,033.8	961.0
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									395.1	206.2	267.1
Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									$ 90.0	$ 155.3	$ 51.2