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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense, period of remediation, years 5 years    
Estimated liability costs of remediation $ 916,000 $ 916,000  
Estimated liability costs of remediation, current $ 50,000 50,000  
Estimated liability, measurement period, years 15 years    
Royalty Expense $ 2,189,000 2,278,000 $ 1,837,000
Future minimum royalty payment for period, 2016 643,000    
Future minimum royalty payment for period, 2017 634,000    
Future minimum royalty payment for period, 2018 659,000    
Future minimum royalty payment for period, 2019 659,000    
Office and Manufacturing facilities operating lease expense 736,000 $ 856,000 $ 657,000
Future minimum rental payments for 2016 424,000    
Future minimum rental payments for 2017 161,000    
Future minimum rental payments for 2018 161,000    
Future minimum rental payments for 2019 85,200    
Future minimum rental payments for 2020 and later 62,400    
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense 47,000    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense $ 56,000