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INCOME TAX (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Nov. 30, 2014
Nov. 30, 2013
Aug. 31, 2014
Effective income tax rate from continuing operations 28.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations 30.90%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Statutory income tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Effective income tax rate from discontinued operations 1.00%cmc_EffectiveTaxRateFromDiscontinuedOperation 42.10%cmc_EffectiveTaxRateFromDiscontinuedOperation  
Net income tax payments $ 11.6us-gaap_IncomeTaxesPaidNet $ 1.4us-gaap_IncomeTaxesPaidNet  
Unrecognized income tax benefits 27.3us-gaap_UnrecognizedTaxBenefits   27.3us-gaap_UnrecognizedTaxBenefits
Possible unrecognized income tax benefits decrease during the next twelve months 16.9us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit    
Unrecognized income tax benefits that would reduce provisions for income taxes 2.7us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate    
Howell      
Pre-tax gain   $ 23.8us-gaap_DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cmc_HowellMember
 
POLAND      
Foreign income tax rate 19.00%cmc_Foreignincometaxrate
/ us-gaap_StatementGeographicalAxis
= country_PL