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Defined Contribution Benefit Plan - Additional Information (Detail) (USD $)
12 Months Ended
May 31, 2013
May 31, 2012
May 31, 2011
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution 401(K) benefit expense $ 863,000 $ 760,000 $ 733,000
First 3% employees deferred amount
     
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution plan employer matching contribution 100.00%    
Next 2% employees deferred amount
     
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution plan employer matching contribution 50.00%