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Post Retirement Benefits (Details) (USD $)
0 Months Ended 12 Months Ended
Aug. 19, 2010
Dec. 31, 2014
Plan
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2010
Dec. 31, 2015
Postretirement benefits:            
Number of 401k plans   3cmt_NumberOf401KPlans        
Payments for post retirement benefits $ 1,257,000us-gaap_OtherPostretirementBenefitsPayments          
Amounts that will be amortized from accumulated other comprehensive income in necxt fiscal year:            
Interest cost   277,000us-gaap_DefinedBenefitPlanInterestCost 332,000us-gaap_DefinedBenefitPlanInterestCost 364,000us-gaap_DefinedBenefitPlanInterestCost    
Postretirement benefits            
Postretirement benefits:            
Decrease in post retirement obligations         10,282,000us-gaap_IncreaseDecreaseInPensionAndPostretirementObligations
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
 
Amortization period         20 years  
Change in net periodic benefit cost due to plan amendment   496,000cmt_DefinedBenefitPlanNetPeriodicBenefitCostAnnualChangeDueToPlanAmmendment
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Actuarial gain (loss)   2,726,000us-gaap_DefinedBenefitPlanActuarialGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
(2,917,000)us-gaap_DefinedBenefitPlanActuarialGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
     
Prior service credit not yet recognized   3,934,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
7,109,000us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetPriorServiceCostsCreditsNotYetRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
     
Amounts that will be amortized from accumulated other comprehensive income in necxt fiscal year:            
Amortization of prior service credit, net of tax   (496,000)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Amortization of loss, net of tax   (169,000)us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Interest cost   277,000us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
332,000us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
     
Assumed health care cost trend rates:            
Health care cost trend rate assumed for next fiscal year   7.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
7.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
     
Ultimate health care cost trend rate   5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
     
Effect of changing the health care cost trend rate by one-percentage point for each future year:            
Effect on total of service and interest cost components, 1 - percentage point increase   23,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnServiceAndInterestCostComponents
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Effect on total of service and interest cost components, 1 - percentage point decrease   (38,000)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnServiceAndInterestCostComponents1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Effect on post retirement benefit obligation, 1 - percentage point increase   1,060,000us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointIncreaseOnAccumulatedPostretirementBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Effect on post retirement benefit obligation, 1 - percentage point decrease   (1,250,000)us-gaap_DefinedBenefitPlanEffectOfOnePercentagePointDecreaseOnAccumulatedPostretirementBenefitObligation1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Forecast | Postretirement benefits            
Postretirement benefits:            
Change in net periodic benefit cost due to plan amendment           496,000cmt_DefinedBenefitPlanNetPeriodicBenefitCostAnnualChangeDueToPlanAmmendment
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Actuarial gain (loss)           (11,000)us-gaap_DefinedBenefitPlanActuarialGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Amounts that will be amortized from accumulated other comprehensive income in necxt fiscal year:            
Interest cost           $ 316,000us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember