Revenue from Contracts with Customers - Disaggregated Sales and Other Operating Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 382.7	$ 391.3
AM USA
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	195.8	197.6
AM Brazil
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	8.5	9.7
AK Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	107.9	104.1
U.S. Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	58.9	54.1
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	11.6	25.8
Cokemaking
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	351.5	344.5
Energy
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	13.1	13.8
Logistics
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	8.9	22.0
Operating and licensing fees
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	8.5	9.7
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 0.7	$ 1.3