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Document and Entity Information - shares
 6 Months Ended
Feb. 28, 2017
Mar. 23, 2017
Document And Entity Information [Abstract]    
Trading Symbol CMC  
Entity Registrant Name COMMERCIAL METALS CO  
Entity Central Index Key 0000022444  
Document Type 10-Q  
Document Period End Date Feb. 28, 2017  
Amendment Flag false  
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus Q2  
Current Fiscal Year End Date --08-31  
Entity Tax Identification Number 750725338  
Entity Filer Category Large Accelerated Filer  
Entity Current Reporting Status Yes  
Entity Common Stock, Shares Outstanding   115,778,772