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Inventories (Detail) (USD $)
In Thousands, unless otherwise specified
Feb. 28, 2013
May 31, 2012
Working Capital Detail [Line Items]    
Inventories $ 100,282 $ 99,441
Lifo Cost
   
Working Capital Detail [Line Items]    
Finished products 5,111 7,664
Work in process 11,923 12,505
Raw materials 16,638 9,449
Total inventories at LIFO cost 33,672 29,618
Average Cost
   
Working Capital Detail [Line Items]    
Finished products 21,475 23,451
Raw materials 143 229
Parts and supplies 44,992 46,143
Total inventories at average cost $ 66,610 $ 69,823