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Summary of Significant Accounting Policies - Concentration Risk (Details)	12 Months Ended
	Dec. 31, 2014 Customer	Dec. 31, 2013	Dec. 31, 2012
Accounts receivable | Customer concentration risk
Concentration risk:
Concentration Risk, Accounts Receivable, Number of major customers	4cmt_NumberofmajorcustomersAccountsReceivable / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Concentration risk percentage	90.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember	87.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Sales | Customer concentration risk
Concentration risk:
Number of major customers, Sales	4cmt_ConcentrationRiskSalesNumberofSignificantCustomers / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Concentration risk percentage	87.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember	86.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember	86.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Sales | Geographic concentration risk | Mexico and Canada
Concentration risk:
Concentration risk percentage	30.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_GeographicConcentrationRiskMember / us-gaap_StatementGeographicalAxis = cmt_MexicoAndCanadaMember	34.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_GeographicConcentrationRiskMember / us-gaap_StatementGeographicalAxis = cmt_MexicoAndCanadaMember	29.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = cmt_SalesRevenueTotalMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_GeographicConcentrationRiskMember / us-gaap_StatementGeographicalAxis = cmt_MexicoAndCanadaMember
Number of employees, geographic area | Labor force concentration risk
Concentration risk:
Concentration risk, number of employees	1,490cmt_ConcentrationRiskNumberOfEmployees / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_NumberOfEmployeesGeographicAreaMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_LaborForceConcentrationRiskMember
Number of employees, geographic area | Labor force concentration risk | Mexico
Concentration risk:
Concentration risk, number of employees	776cmt_ConcentrationRiskNumberOfEmployees / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_NumberOfEmployeesGeographicAreaMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_LaborForceConcentrationRiskMember / us-gaap_StatementGeographicalAxis = country_MX
Number of employees, geographic area | Labor force concentration risk | United States
Concentration risk:
Concentration risk, number of employees	714cmt_ConcentrationRiskNumberOfEmployees / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_NumberOfEmployeesGeographicAreaMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_LaborForceConcentrationRiskMember / us-gaap_StatementGeographicalAxis = country_US
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk
Concentration risk:
Concentration risk, number of employees	1,490cmt_ConcentrationRiskNumberOfEmployees / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_UnionizedEmployeesConcentrationRiskMember
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | International Association of Machinists and Aerospace Workers
Concentration risk:
Concentration risk, number of employees	333cmt_ConcentrationRiskNumberOfEmployees / us-gaap_CollectiveBargainingArrangementAxis = cmt_InternationalAssociationOfMachinistsAndAerospaceWorkersMember / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_UnionizedEmployeesConcentrationRiskMember
Workforce subject to collective bargaining arrangements | Unionized employees concentration risk | Sindicato de Jorneleros y Obreros
Concentration risk:
Concentration risk, number of employees	672cmt_ConcentrationRiskNumberOfEmployees / us-gaap_CollectiveBargainingArrangementAxis = cmt_SindicatoDeJornelerosYObrerosMember / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_UnionizedEmployeesConcentrationRiskMember

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: 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=28364263&loc=d3e13537-108611



Reference 2: 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=6927468&loc=d3e6404-108592



Reference 3: 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=28364263&loc=d3e13531-108611



Reference 4: 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=6927468&loc=d3e6351-108592

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