Segment Information - Summary of sale by product and service (Details) - item	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Information
Number of reportable segments	1	1	1
Sales (as a percent)	100.00%	100.00%	100.00%
Carbon steel
Segment Information
Sales (as a percent)	53.00%	52.00%	52.00%
Aluminum
Segment Information
Sales (as a percent)	19.00%	19.00%	20.00%
Stainless steel
Segment Information
Sales (as a percent)	14.00%	14.00%	14.00%
Alloy
Segment Information
Sales (as a percent)	6.00%	6.00%	5.00%
Toll processing and logistics
Segment Information
Sales (as a percent)	4.00%	4.00%	4.00%
Other
Segment Information
Sales (as a percent)	4.00%	5.00%	5.00%