Capital Stock Capital Stock (Narrative) (Details) (USD $)	12 Months Ended
	Aug. 31, 2012
Stockholders' Equity Note [Abstract]
Treasury Stock, Shares, Acquired	0
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	8,259,647
Preferred Stock, Par or Stated Value Per Share	$ 1.00
Preferred Stock, Shares Authorized	2,000,000
Shareholders' Warrant or Right Plan [Line Items]
Conditions to Be Followed for Rights to Be Exercisable or Traded Separately	The Rights initially are represented by and trade with shares of CMC common stock. The Rights will not become exercisable or trade separately from shares of CMC common stock unless one or both of the following conditions are met: a public announcement that a person or group has acquired 10% or more of CMC common stock (including in the form of synthetic ownership through derivative positions), or a tender or exchange offer is made for 10% or more of CMC common stock.
Condition for Rights to Be Redeemed By Company	The Rights may be redeemed by CMC for $0.001 per Right at any time until the tenth day following the first public announcement of the acquisition of beneficial ownership of 10% of CMC common stock. The Rights Plan exempts any person or group owning 10% or more of CMC common stock as of the announcement of the Rights Plan. However, the Rights also will be exercisable if a person or group that owns 10% or more of CMC common stock as of such announcement acquires any additional shares (including through derivatives, but other than pursuant to a dividend or distribution paid or made by CMC or pursuant to a stock split or reclassification).
Stockholders Right Plan Expiration	Aug. 01, 2014
Shareholder Rights Plan [Member]
Shareholders' Warrant or Right Plan [Line Items]
Discount on Company's Common Stock or an Acquirers Stock	50.00%
Value Per Rights in Class of Warrant or Right Number of Securities	0.001
Shareholder Rights Plan [Member] | Minimum [Member]
Shareholders' Warrant or Right Plan [Line Items]
Stockholder Rights Plan Exercise Trigger Threshold Percentage	10.00%
Series B Junior Participating Preferred Stock [Member]
Shareholders' Warrant or Right Plan [Line Items]
Number of shares exercisable for stock purchase right	0.001
Class of Warrant or Right, Exercise Price of Warrants or Rights	70