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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
May 31, 2020
May 31, 2019
May 31, 2018
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 2,524,000 2,795,000 $ 2,876,000
Future minimum royalty payment for period, 2021 182,000    
Future minimum royalty payment for period, 2022 110,000    
Future minimum royalty payment for period, 2023 105,000    
Future minimum royalty payment for period, 2024 105,000    
Future minimum royalty payment for period, 2025 105,000    
Office and Manufacturing facilities operating lease expense 1,373,000 $ 1,633,000 $ 1,083,000
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense 38,000    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense $ 131,000