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Summary of Significant Accounting Policies - Concentration Risk (Details)	12 Months Ended
	Dec. 31, 2017 Employee Customer	Dec. 31, 2016	Dec. 31, 2015
Sales | Customer Concentration Risk [Member]
Concentration risk:
Number of major customers, Sales | Customer	5
Concentration Risk, Percentage	84.00%	85.00%	85.00%
Sales | Geographic concentration risk | Mexico and Canada
Concentration risk:
Concentration Risk, Percentage	36.00%	32.00%	35.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member]
Concentration risk:
Concentration Risk, Accounts Receivable, Number of major customers | Customer	5
Concentration Risk, Percentage	84.00%	85.00%
Number of employees, geographic area | Labor force concentration risk
Concentration risk:
Concentration risk, number of employees	1,304
Number of employees, geographic area | Labor force concentration risk | United States
Concentration risk:
Concentration risk, number of employees	596
Number of employees, geographic area | Labor force concentration risk | Mexico
Concentration risk:
Concentration risk, number of employees	708
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk
Concentration risk:
Concentration risk, number of employees	1,304
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | International Association of Machinists and Aerospace Workers
Concentration risk:
Concentration risk, number of employees	248
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | Sindicato de Jorneleros y Obreros
Concentration risk:
Concentration risk, number of employees	611

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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