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Commitments and Contingencies - Additional Information (Detail) - USD ($)
 12 Months Ended
May 31, 2017
May 31, 2016
May 31, 2015
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 $ 54,000 54,000  
Royalty Expense 2,659,000 1,969,000 $ 2,189,000
Future minimum royalty payment for period, 2018 625,000    
Future minimum royalty payment for period, 2019 659,000    
Future minimum royalty payment for period, 2020 666,000    
Future minimum royalty payment for period, 2021 674,000    
Future minimum royalty payment for period, 2022 597,000    
Office and Manufacturing facilities operating lease expense 729,000 $ 662,000 $ 736,000
Future minimum rental payments for 2018 591,000    
Future minimum rental payments for 2019 292,000    
Future minimum rental payments for 2020 88,000    
Future minimum rental payments for 2021 87,000    
Future minimum rental payments for 2022 and later 91,000    
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense 38,000    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Environmental remediation expense $ 57,000