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Post Retirement Benefits - Multi-employer Plans (Narrative) (Details)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Multiemployer plans:    
Multiemployer Plans, Plan Benefits, Description a multi-employer defined benefit pension plan, health care and life insurance benefits  
Multiemployer Plan, Type [Extensible Enumeration] Pension Plan [Member]  
Multiemployer Plan, Sponsor Location [Extensible Enumeration] us-gaap:DomesticPlanMember  
Multiemployer Plan, Pension, Significant, Annual Report Date Dec. 31, 2020  
Multiemployer Plan, Pension, Significant, Certified Zone Status, Date Dec. 31, 2020  
Pension Protection Act Zone Status Red Red
Multiemployer Plan, Pension, Significant, Funded Status [Fixed List] Less than 65 percent  
Multiemployer Plan, Pension, Significant, Employer Contribution Exceeds Five Percent [true false] false  
Multiemployer Plans, Minimum Contribution, Description Under the terms of the collective-bargaining agreement, the Company is required to make contributions to the plan for each hour worked up to a maximum of 40 hours per person, per week at $1.55 per hour from August 10, 2019 through August 6, 2022. The Company is paying a surcharge of $2.40.  
Multiemployer Plan, Pension, Significant, Collective-Bargaining Arrangement [Extensible Enumeration] cmt:InternationalAssociationofMachinistsAndAerospaceWorkersMember