Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 434.1	$ 470.9	$ 870.1	$ 959.3
Cliffs Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	250.7	286.9	550.0	604.2
U.S. Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	60.3	70.1	120.3	142.1
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	123.1	113.9	199.8	213.0
Cokemaking
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	397.2	429.3	788.5	876.0
Energy
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	12.5	11.5	25.3	23.4
Logistics
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	13.9	19.8	36.0	40.1
Operating and licensing fees
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	8.6	9.1	16.4	17.4
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 1.9	$ 1.2	$ 3.9	$ 2.4