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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
May 31, 2016
May 31, 2015
May 31, 2014
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, measurement period, years 15 years    
Estimated liability costs of remediation, current $ 60,000 60,000  
Royalty Expense 1,969,000 2,189,000 $ 2,278,000
Future minimum royalty payment for period, 2017 572,000    
Future minimum royalty payment for period, 2018 597,000    
Future minimum royalty payment for period, 2019 597,000    
Future minimum royalty payment for period, 2020 593,000    
Future minimum royalty payment for period, 2021 586,000    
Office and Manufacturing facilities operating lease expense 662,000 $ 736,000 $ 856,000
Future minimum rental payments for 2017 541,000    
Future minimum rental payments for 2018 290,000    
Future minimum rental payments for 2019 195,000    
Future minimum rental payments for 2020 96,000    
Future minimum rental payments for 2021 and later 22,000    
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense 47,000    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense $ 57,000