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12. STOCKHOLDERS' EQUITY, COMPREHENSIVE INCOME AND STOCK-BASED COMPENSATION (Details 2) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Feb. 28, 2019
Feb. 28, 2018
Feb. 28, 2019
Feb. 28, 2018
Stock Options        
Expected term (in years) 5 years 4 years 5 years 4 years
Volatility 69.00% 73.00% 72.00% 77.00%
Risk-free interest rate 2.77% 2.31% 2.83% 1.81%
Weighted average grant date fair value $ 1.01 $ 1.53 $ 1.34 $ 2.17
ESPP        
Expected dividend     $ 0  
Estimated forfeiture rate     0.00%  
Weighted average grant date fair value     $ 1.15  
ESPP | Minimum        
Expected term (in years)     6 months  
Volatility     48.00%  
Risk-free interest rate     2.40%  
ESPP | Maximum        
Expected term (in years)     2 years  
Volatility     64.00%  
Risk-free interest rate     2.82%