Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014 employees	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 employees	Dec. 31, 2013	Dec. 31, 2012
Concentration Of Risk [Line Items]
Net sales	$ 1,997.6	$ 1,593.8	$ 1,530.8	$ 1,383.5	$ 1,464.8	$ 1,331.3	$ 1,404.5	$ 1,369.8	$ 6,505.7	$ 5,570.4	$ 5,933.7
Number of Employees	8,000								8,000
Stainless And Electrical [Member]
Concentration Of Risk [Line Items]
Net sales									1,836.5	1,705.3	1,898.9
Carbon [Member]
Concentration Of Risk [Line Items]
Net sales									4,423.3	3,643.4	3,789.6
Tubular [Member]
Concentration Of Risk [Line Items]
Net sales									231.4	220.7	243.6
Other Product Lines [Member]
Concentration Of Risk [Line Items]
Net sales									14.5	1.0	1.6
Workforce Subject to Collective Bargaining Arrangements [Member]
Concentration Of Risk [Line Items]
Number of Employees	6,200								6,200
Customer Concentration Risk [Member] | Automotive Market [Member] | Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)									53.00%	51.00%	45.00%
Customer Concentration Risk [Member] | Infrastructure and Manufacturing Market [Member] | Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)									18.00%	20.00%	23.00%
Customer Concentration Risk [Member] | Distributors and Converters Market [Member] | Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)									29.00%	29.00%	32.00%
Customer Concentration Risk [Member] | Foreign Customers [Member] | Sales Revenue, Goods, Net [Member]
Concentration Of Risk [Line Items]
Net sales									$ 755.4	$ 708.0	$ 856.7
Customer Concentration Risk [Member] | Customer Concentration Risk [Member] | Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)									14.00%	11.00%
Credit Concentration Risk [Member] | Automotive Market [Member] | Accounts Receivable [Member]
Concentration Of Risk [Line Items]
Percentage of total (in hundredths)									43.00%	41.00%
IAM National Pension Fund's National Pension Plan [Member] | Zanesville Works [Member] | Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Concentration Of Risk [Line Items]
Number of Employees	130								130
Steelworkers Pension Trust [Member] | Mountain State Carbon, LLC [Member] | Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member]
Concentration Of Risk [Line Items]
Number of Employees	215								215
Forward Contracts [Member]
Concentration Of Risk [Line Items]
Forward contracts duration (in months)									0 years 24 months