Description of the Business and Significant Accounting Policies - Segments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016 segment item	Sep. 30, 2015
Segment Reporting Information
Number of reporting segments | segment			3
Steel Operations
Segment Reporting Information
Number of downstream coating facilities			10
Percentage of external net sales	74.00%	69.00%	72.00%	69.00%
Metals Recycling Operations
Segment Reporting Information
Percentage of external net sales	15.00%	18.00%	15.00%	19.00%
Steel Fabrication Operations
Segment Reporting Information
Percentage of external net sales	8.00%	9.00%	9.00%	8.00%
Number of New Millennium Building Systems' joist and deck plants			8