NEW MARKETS TAX CREDIT TRANSACTIONS (Details) $ in Millions	12 Months Ended
Aug. 31, 2018 USD ($)
Micro Mill
Income Tax Contingency [Line Items]
Ending Restricted Cash	$ 0.5
Spooler Project
Income Tax Contingency [Line Items]
Ending Restricted Cash	0.0
T-post Shop
Income Tax Contingency [Line Items]
Ending Restricted Cash	7.9
Fund 156 \| Micro Mill
Income Tax Contingency [Line Items]
USBCDC Capital Contribution	17.7
Commonwealth Loan	$ 35.3
Commonwealth Loan Rate (Percent)	1.08%
QEI to CDE	$ 51.5
Fund 156 \| Micro Mill \| Qualified Equity Investment Loans \| CMC Steel Oklahoma, LLC
Income Tax Contingency [Line Items]
CDE Loan	50.7
Fund 249 \| Spooler Project
Income Tax Contingency [Line Items]
USBCDC Capital Contribution	6.7
Commonwealth Loan	$ 14.0
Commonwealth Loan Rate (Percent)	1.39%
QEI to CDE	$ 20.0
Fund 249 \| Spooler Project \| Qualified Equity Investment Loans \| CMC Steel Oklahoma, LLC
Income Tax Contingency [Line Items]
CDE Loan	$ 19.4
Fund 219
Income Tax Contingency [Line Items]
Period at end of which Company may be entitled or obligated to repurchase interest in the fund	8 years
Fund 219 and Fund 222 \| T-post Shop
Income Tax Contingency [Line Items]
USBCDC Capital Contribution	$ 5.0
Commonwealth Loan	$ 10.4
Commonwealth Loan Rate (Percent)	1.16%
QEI to CDE	$ 15.0
Fund 219 and Fund 222 \| T-post Shop \| Qualified Equity Investment Loans \| CMC Steel Oklahoma, LLC
Income Tax Contingency [Line Items]
CDE Loan	$ 14.7
Fund 156 and Fund 249
Income Tax Contingency [Line Items]
Period at end of which Company may be entitled or obligated to repurchase interest in the fund	7 years
Fund 222 \| Qualified Equity Investment Loans
Income Tax Contingency [Line Items]
CDE Loan	$ 2.1

Capital Contribution Received Under New Markets Tax Program

No definition available.

Name:	cmc_CapitalContributionReceivedUnderNewMarketsTaxProgram
Namespace Prefix:	cmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Loan Receivable, Stated Interest Rate

+ References

No definition available.

+ Details

Name:	cmc_LoanReceivableStatedInterestRate
Namespace Prefix:	cmc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period at End of Which Company may be Entitled or Obligated to Repurchase Interest in the Fund

+ References

No definition available.

+ Details

Name:	cmc_PeriodatEndofWhichCompanymaybeEntitledorObligatedtoRepurchaseInterestintheFund
Namespace Prefix:	cmc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Qualified Equity Investment Into Community Development Entities

+ References

No definition available.

+ Details

Name:	cmc_QualifiedEquityInvestmentIntoCommunityDevelopmentEntities
Namespace Prefix:	cmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_IncomeTaxContingencyLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Including the current and noncurrent portions, aggregate carrying value as of the balance sheet date of loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16)(a)(2))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_LoansPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(2))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=98513485&loc=SL98516268-108586

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(1)(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3044-108585

+ Details

Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=cmc_MicroMillMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=cmc_SpoolerProjectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=cmc_PostShopProjectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmc_Fund156Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cmc_QualifiedEquityInvestmentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=cmc_CMCSteelOklahomaLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmc_Fund249Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmc_Fund219Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmc_Fund219andFund222Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmc_Fund156andFund249Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=cmc_Fund222Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: