Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 490.1	$ 520.4	$ 1,449.4	$ 1,542.6
Cliffs Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	311.5	338.7	915.7	1,000.8
U.S. Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	72.0	79.2	214.1	230.4
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	106.6	102.5	319.6	311.4
Cokemaking
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	446.2	481.3	1,322.2	1,418.5
Energy
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	12.7	12.5	36.1	36.5
Logistics
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	20.8	15.4	60.9	55.6
Operating and licensing fees
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	8.8	9.1	26.2	25.8
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 1.6	$ 2.1	$ 4.0	$ 6.2