Dispositions (Narrative) (Details) (USD $)	3 Months Ended	9 Months Ended	3 Months Ended
May 31, 2013	May 31, 2012	May 31, 2013	May 31, 2012	Nov. 30, 2012 Trinecke Zelezarny [Member]	Nov. 30, 2011 Joist And Deck Operations [Member] properties
Percentage of ownership before transaction					11.00%
Business divestiture disposal price					$ 29,000,000
Cost-method investments, pre-tax gains	$ 0	$ 0	$ 26,088,000	$ 0	$ 26,100,000
Number of properties sold						2

This represents disposal price of a business unit or asset during divestiture.

No definition available.

Name:	cmc_BusinessDivestitureDisposalPrice
Namespace Prefix:	cmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

It represents number of real properties sold during period.

+ References

No definition available.

+ Details

Name:	cmc_NumberOfPropertiesSoldDuringPeriod
Namespace Prefix:	cmc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

For investments in debt and equity securities accounted for at cost, the excess of net sale proceeds over the carrying amount of investments disposed of during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph c, d

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(c))

-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

+ Details

Name:	us-gaap_CostMethodInvestmentsRealizedGains
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of subsidiary's or equity investee's stock owned by parent company before stock transaction.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section H

+ Details

Name:	us-gaap_SaleOfStockPercentageOfOwnershipBeforeTransaction
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration