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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
May 31, 2018
May 31, 2017
May 31, 2016
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 2,876,000 2,659,000 $ 1,969,000
Future minimum royalty payment for period, 2019 634,000    
Future minimum royalty payment for period, 2020 641,000    
Future minimum royalty payment for period, 2021 649,000    
Future minimum royalty payment for period, 2022 572,000    
Future minimum royalty payment for period, 2023 568,000    
Office and Manufacturing facilities operating lease expense 799,000 729,000 $ 662,000
Future minimum rental payments for 2019 498,000    
Future minimum rental payments for 2020 86,000    
Future minimum rental payments for 2021 108,000    
Future minimum rental payments for 2022 141,000    
Future minimum rental payments for 2023 and later 73,000    
Corrective Measures Study [Member]      
Commitments and Contingencies Disclosure [Line Items]      
Estimated liability costs of remediation, current 45,000 $ 45,000  
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense 38,000    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense $ 74,000