Revenue from Contracts with Customers - Disaggregated Sales and Other Operating Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 302.2	$ 404.3	$ 1,022.9	$ 1,203.1
AM USA
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	154.2	196.8	530.3	586.4
AM Brazil
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	7.1	9.5	22.8	29.2
AK Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	79.0	109.7	276.7	326.5
U.S. Steel
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	49.4	68.9	159.3	191.9
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	12.5	19.4	33.8	69.1
Cokemaking
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	275.8	363.9	937.3	1,073.0
Energy
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	11.2	13.7	36.3	40.4
Logistics
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	8.0	16.0	24.0	57.4
Operating and licensing fees
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	7.1	9.6	22.8	29.3
Other
Disaggregation of Revenue [Line Items]
Sales and other operating revenue	$ 0.1	$ 1.1	$ 2.5	$ 3.0