Working Capital - Additional Information (Detail) (USD $)	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Working Capital Detail [Line Items]
Working capital	$ 208,000,000	$ 249,200,000
Allowances for doubtful receivables	2,500,000	3,900,000
Provisions for bad debts	500,000	3,300,000	2,000,000
Uncollectible accounts written off	1,900,000	2,900,000	600,000
Cost of product sold	601,256,000	596,510,000	562,066,000
Average Cost
Working Capital Detail [Line Items]
Amount inventory values would be higher by average cost method	37,500,000	44,600,000
Cost of product sold	$ 4,100,000	$ 1,200,000	$ 1,800,000