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Defined Contribution Benefit Plan - Additional Information (Detail) - USD ($)
 12 Months Ended
May 31, 2018
May 31, 2017
May 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution 401(K) benefit expense $ 1,325,000 $ 1,259,000 $ 1,188,000
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%