Pension and Other Postretirement Benefits - Multiemployer Plans (Details 6) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) employees	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer | $		$ 22.8	$ 21.9	$ 24.7
Number of Employees		9,300
Steelworkers Pension Trust [Member]
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer | $		$ 3.9	4.2	6.3
IAM National Pension Fund's National Pension Plan [Member]
Multiemployer Plans [Line Items]
Multiemployer Plan, Contributions by Employer | $		$ 18.9	$ 17.7	$ 18.4
Ashland Works [Member] | Steelworkers Pension Trust [Member]
Multiemployer Plans [Line Items]
Number of Employees		10
AK Tube Walbridge [Member] | Steelworkers Pension Trust [Member]
Multiemployer Plans [Line Items]
Number of Employees		100
Mansfield Works [Member] | Steelworkers Pension Trust [Member]
Multiemployer Plans [Line Items]
Number of Employees		300
Middletown Works [Member] | IAM National Pension Fund's National Pension Plan [Member]
Multiemployer Plans [Line Items]
Number of Employees		1,750
Zanesville Works [Member] | IAM National Pension Fund's National Pension Plan [Member]
Multiemployer Plans [Line Items]
Number of Employees		100
Butler Works [Member] | IAM National Pension Fund's National Pension Plan [Member]
Multiemployer Plans [Line Items]
Number of Employees		1,100
Forecast [Member] | IAM National Pension Fund's National Pension Plan [Member]
Multiemployer Plans [Line Items]
Potential increase to contributions to multiemployer plans | $	$ 2.0