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Summary of Significant Accounting Policies - Concentration Risks (Narrative) (Details) - Employees	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Sales Revenue, Total | Customer Concentration Risk
Concentration risk
Concentration Risk, Customer	Navistar, Volvo, PACCAR, BRP and UFP
Concentration Risk, Percentage	70.00%	70.00%	70.00%
Sales Revenue, Total | Customer Concentration Risk | Mexico and Canada
Concentration risk
Concentration Risk, Percentage	35.00%	34.00%	32.00%
Accounts Receivable | Customer Concentration Risk
Concentration risk
Concentration Risk, Percentage	64.00%	56.00%
Number of employees, geographic area | Labor force concentration risk
Concentration risk
Entity Number of Employees	1,617
Number of employees, geographic area | Labor force concentration risk | United States
Concentration risk
Entity Number of Employees	679
Number of employees, geographic area | Labor force concentration risk | Mexico
Concentration risk
Entity Number of Employees	722
Number of employees, geographic area | Labor force concentration risk | Canada
Concentration risk
Entity Number of Employees	216
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | United States | International Association of Machinists and Aerospace Workers
Concentration risk
Entity Number of Employees	518
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | Mexico | Sindicato de Jorneleros y Obreros
Concentration risk
Entity Number of Employees	534
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | Mexico | Sindicato de trabajadores de la industria metalica y del comercio del estado de Nuevo Leon Presidente Benito Juarez Garcia C.T.M.
Concentration risk
Entity Number of Employees	73
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | Canada | United Food & Commercial Workers Canada
Concentration risk
Entity Number of Employees	191

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Description of risks that arise due to the volume of business transacted with a particular customer. At a minimum, the description informs financial statement users of the general nature of the risk, but excludes "Information about Major Customers" that may be disclosed elsewhere (for instance, segment disclosures).

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