Quarterly report pursuant to Section 13 or 15(d)

Commitments and Contingencies (Details Textual)

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Commitments and Contingencies (Details Textual) (USD $)
9 Months Ended
Feb. 28, 2013
May 31, 2012
Commitments and Contingencies (Textual)    
Lines of credit amount outstanding $ 0  
Irrevocable standby letters of credit amount available 38,800,000 46,800,000
Expiration date range of letters of credit July 2013 and December 2013  
Irrevocable stand by letters of credit amount outstanding 0  
Commitments to purchase capital assets $ 4,400,000 $ 7,600,000