Summary of Significant Accounting Policies (Details2)	12 Months Ended
	Dec. 31, 2019 USD ($) employees	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Concentration Of Risk [Line Items]
Number of Employees	9,300
Goodwill, Impairment Loss | $	$ 0	$ 0	$ 0
Operating Lease, Right-of-Use Asset | $	244,500,000	$ 0
Operating Lease, Liability | $	$ 261,700,000
Workforce Subject to Collective Bargaining Arrangements [Member]
Concentration Of Risk [Line Items]
Number of Employees	5,600
Workforce Subject to Collective Bargaining Arrangements [Member] | Mountain State Carbon [Member]
Concentration Of Risk [Line Items]
Number of Employees	220
Workforce Subject to Collective Bargaining Arrangements [Member] | Zanesville Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	100
Workforce Subject to Collective Bargaining Arrangements [Member] | Butler Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	1,100
Workforce Subject to Collective Bargaining Arrangements [Member] | Coshocton Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	310
Workforce Subject to Collective Bargaining Arrangements [Member] | Middletown Works [Member]
Concentration Of Risk [Line Items]
Number of Employees	1,750
Workforce Subject to Collective Bargaining Arrangements [Member] | AK Tube Walbridge [Member]
Concentration Of Risk [Line Items]
Number of Employees	100
Customer Concentration Risk [Member] | Revenue Benchmark [Member]
Concentration Of Risk [Line Items]
Number Of Major Customers	2	2	1
Customer Concentration Risk [Member] | Major Customer A [Member] | Revenue Benchmark [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	11.00%	11.00%	12.00%
Customer Concentration Risk [Member] | Major Customer B [Member] | Revenue Benchmark [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	11.00%	10.00%
Credit Concentration Risk [Member] | Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Number Of Major Customers	1	1
Credit Concentration Risk [Member] | Automotive Market [Member] | Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	62.00%	52.00%
Credit Concentration Risk [Member] | Major Customer A [Member] | Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)	15.00%	13.00%
Maximum [Member] | Commodity [Member]
Concentration Of Risk [Line Items]
Derivative contracts duration (in months)	2 years
Maximum [Member] | Foreign Exchange Contract [Member]
Concentration Of Risk [Line Items]
Derivative contracts duration (in months)	3 years
Machinery and equipment [Member] | Maximum [Member]
Concentration Of Risk [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Machinery and equipment [Member] | Minimum [Member]
Concentration Of Risk [Line Items]
Property, Plant and Equipment, Useful Life	2 years
Accounting Standards Update 2016-02 [Member]
Concentration Of Risk [Line Items]
Operating Lease, Right-of-Use Asset | $				$ 291,100,000
Operating Lease, Liability | $				$ 291,100,000.0