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Post Retirement Benefits - Multiemployer Plans (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Aug. 11, 2014
Aug. 10, 2013
Aug. 12, 2012
Aug. 09, 2016
Aug. 09, 2015
Multiemployer plans:                
Certified zone status, percent funded limit for yellow status 80.00%cmt_MultiemployerPlansCertifiedZoneStatusPercentFundedLimitForYellowStatus              
Certified zone status, percent funded limit for green status 80.00%cmt_MultiemployerPlansCertifiedZoneStatusPercentFundedLimitForGreenStatus              
Certified zone status, percent funded limit for red status 65.00%cmt_MultiemployerPlansCertifiedZoneStatusPercentFundedLimitForRedStatus              
Multi-employer plan contributions $ 719,000us-gaap_MultiemployerPlanPeriodContributions [1] $ 464,000us-gaap_MultiemployerPlanPeriodContributions [1] $ 443,000us-gaap_MultiemployerPlanPeriodContributions [1]          
Multiemployer plan, postretirement benefit                
Multiemployer plans:                
Maximum percentage of total contribution 5.00%cmt_MultiemployerPlansMaximumPercentageOfTotalContribution
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		              
Multiemployer plans, collective-bargaining arrangement, maximum hours of work per week for employer contribution 40 hours              
Multiemployer plans, collective-bargaining arrangement, rate of employer contribution       1.30cmt_MultiemployerPlansCollectiveBargainingArrangementRateOfEmployerContribution
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		 1.25cmt_MultiemployerPlansCollectiveBargainingArrangementRateOfEmployerContribution
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		 1.20cmt_MultiemployerPlansCollectiveBargainingArrangementRateOfEmployerContribution
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember		    
Forecast | Multiemployer plan, postretirement benefit                
Multiemployer plans:                
Multiemployer plans, collective-bargaining arrangement, rate of employer contribution             1.40cmt_MultiemployerPlansCollectiveBargainingArrangementRateOfEmployerContribution
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 1.35cmt_MultiemployerPlansCollectiveBargainingArrangementRateOfEmployerContribution
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPostretirementBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
[1] The National Pension Plan utilized a five year amortization extension in accordance with § 431(d) of the Internal Revenue Code of 1986 ("the Code") to amortize its losses from 2008. The plan re-certified its zone status after using the amortization provisions of the Code. The Company's contributions to the plan did not represent more than 5% of total contributions to the plan as indicated in the plan's most recently available annual report for the plan year ended December 31, 2013. 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 the following rates: $1.20 per hour from August 8, 2011 through August 12, 2012; $1.25 per hour from August 13, 2012 through August 10, 2013