Document and Entity Information - USD ($)	12 Months Ended
	Aug. 31, 2017	Oct. 24, 2017	Feb. 28, 2017
Document And Entity Information [Abstract]
Trading Symbol	cmc
Entity Registrant Name	COMMERCIAL METALS CO
Entity Central Index Key	0000022444
Document Type	10-K
Document Period End Date	Aug. 31, 2017
Amendment Flag	false
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--08-31
I.R.S. Employer Identification No.	750725338
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		115,840,033
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Minimum Percentage of Share Ownership Deemed Affiliates			10.00%
Entity Public Float			$ 2,420,166,134