Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 475.3	$ 434.1	$ 930.4	$ 870.1
Domestic
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	448.0	425.5	879.0	853.7
International
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	27.3	8.6	51.4	16.4
Cliffs Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	290.0	250.7	555.0	550.0
U.S. Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	64.4	60.3	131.3	120.3
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	120.9	123.1	244.1	199.8
Cokemaking
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	357.5	397.2	707.9	788.5
Energy
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	9.3	12.5	19.8	25.3
Industrial Services
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	97.6	13.9	182.5	36.0
Operating and licensing fees
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	9.4	8.6	17.4	16.4
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 1.5	$ 1.9	$ 2.8	$ 3.9