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Document and Entity Information - shares
 9 Months Ended
May 31, 2019
Jun. 25, 2019
Document And Entity Information [Abstract]    
Entity Registrant Name COMMERCIAL METALS CO  
Entity Central Index Key 0000022444  
Document Type 10-Q  
Document Period End Date May 31, 2019  
Amendment Flag false  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Current Fiscal Year End Date --08-31  
Entity Tax Identification Number 750725338  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   117,924,115
Entity File Number 75-0725338  
Entity Address, Address Line One 6565 N. MacArthur Blvd.  
Entity Address, City or Town Irving  
Entity Address, State or Province Texas  
Entity Address, Postal Zip Code 75039  
City Area Code 214  
Local Phone Number 689-4300