Segment and Geographical Area Information, Narrative (Details) - productionfacilities	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information
Percentage of Employees Covered by Collective Bargaining Agreements	63.00%
Percentage of Employees Covered by Collective Bargaining Agreements Expiring Within One Year	6.00%
Domestic
Segment Reporting Information
Number of production facilities	11
Foreign
Segment Reporting Information
Number of production facilities	1
Customer Concentration Risk | Largest Customer | Sales Revenue, Net | Fabricated Products
Segment Reporting Information
Concentration Risk, Percentage	27.00%	29.00%
Customer Concentration Risk | Largest Customer | Accounts Receivable
Segment Reporting Information
Concentration Risk, Percentage	24.00%		22.00%
Customer Concentration Risk | Second Largest Customer | Sales Revenue, Net | Fabricated Products
Segment Reporting Information
Concentration Risk, Percentage	13.00%	10.00%
Customer Concentration Risk | Second Largest Customer | Accounts Receivable
Segment Reporting Information
Concentration Risk, Percentage	16.00%		14.00%