Deferred Tax Assets and Liabilities (Detail) (USD $)	May 31, 2012	May 31, 2011
Deferred tax assets
Accounts receivable	$ 1,640,000	$ 1,870,000
Inventories	5,323,000	5,932,000
Accrued expenses	30,403,000	29,227,000
Net operating and capital loss carry forwards	21,664,000	22,501,000
Tax credit carry forwards	1,044,000	1,265,000
Stock-based compensation	10,771,000	7,187,000
Derivative contracts	5,134,000	3,761,000
Other	12,000	7,000
Total deferred tax assets	75,991,000	71,750,000
Valuation allowance for deferred tax assets	(22,025,000)	(22,292,000)
Net deferred tax assets	53,966,000	49,458,000
Deferred tax liabilities
Property, plant and equipment	(60,373,000)	(58,606,000)
Undistributed earnings of unconsolidated affiliates	(45,159,000)	(43,947,000)
Other	(666,000)	(583,000)
Total deferred tax liabilities	(106,198,000)	(103,136,000)
Net deferred tax liabilities	$ (52,232,000)	$ (53,678,000)