Segment and Geographical Area Information, Narrative (Details) - productionfacilities	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information
Percentage of employees covered by collective bargaining agreements	62.00%		62.00%
Percentage of employees covered by collective bargaining agreements expiring within one year	0.00%		0.00%
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 percentages (percent)	27.00%	28.00%	27.00%	29.00%
Customer Concentration Risk | Largest Customer | Accounts Receivable
Segment Reporting Information
Concentration percentages (percent)			22.00%		22.00%
Customer Concentration Risk | Second Largest Customer | Sales Revenue, Net | Fabricated Products
Segment Reporting Information
Concentration percentages (percent)	11.00%	12.00%	12.00%	11.00%
Customer Concentration Risk | Second Largest Customer | Accounts Receivable
Segment Reporting Information
Concentration percentages (percent)			15.00%		14.00%