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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
May 31, 2019
May 31, 2018
May 31, 2017
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 $ 100,000 100,000  
Royalty Expense 2,795,000 2,876,000 $ 2,659,000
Future minimum royalty payment for period, 2020 183,000    
Future minimum royalty payment for period, 2021 191,000    
Future minimum royalty payment for period, 2022 114,000    
Future minimum royalty payment for period, 2023 109,000    
Future minimum royalty payment for period, 2024 109,000    
Office and Manufacturing facilities operating lease expense 871,000 $ 799,000 $ 729,000
Future minimum rental payments for 2020 1,112,000    
Future minimum rental payments for 2021 810,000    
Future minimum rental payments for 2022 297,000    
Future minimum rental payments for 2023 101,000    
Future minimum rental payments for 2024 0    
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense 38,000    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense $ 131,000