Fair Values of Company's Derivatives (Detail) (USD $)	Aug. 31, 2012		Aug. 31, 2011
Cash collateral due offset	$ 2,600,000		$ 5,100,000
Commodity Contract [Member] | Other Current Assets [Member]
Commodity derivatives	(1,422,000)	[1],[2]
Commodity derivatives(1)			(3,153,000)	[3]
Quoted Prices In Active Markets for Identical Instruments (Level 1) [Member] | Commodity Contract [Member] | Other Current Assets [Member]
Commodity derivatives	(1,422,000)	[1],[2]
Commodity derivatives(1)	(3,153,000)	[3]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contract [Member] | Other Current Assets [Member]
Commodity derivatives		[1],[2]
Commodity derivatives(1)				[3]
Significant Unobservable Inputs (Level 3) [Member] | Commodity Contract [Member] | Other Current Assets [Member]
Commodity derivatives		[1],[2]
Commodity derivatives(1)				[3]

[1] Gains and losses reported in cost of goods sold

[2] On the consolidated balance sheets, commodity derivative assets and liabilities have been offset by cash collateral due and paid under master netting arrangements which are recorded together in Other Current Assets. The cash collateral offset was $2.6 million at August 31, 2012.

[3] On the consolidated balance sheets, commodity derivative assets and liabilities have been offset by cash collateral due and paid under master netting arrangements which are recorded together in Other Current Assets. The cash collateral offset was $5.1 million at August 31, 2011.