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13. STOCKHOLDERS' EQUITY, COMPREHENSIVE INCOME AND STOCK-BASED COMPENSATION (Details 2) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Nov. 30, 2018
Nov. 30, 2017
Nov. 30, 2018
Nov. 30, 2017
Stock Options        
Expected term (in years) 5 years 4 years 5 years 4 years
Volatility 70.00% 74.00% 72.00% 77.00%
Risk-free interest rate 3.01% 1.92% 2.84% 1.77%
Weighted average grant date fair value $ 1.21 $ 1.93 $ 1.38 $ 2.22
ESPP        
Expected dividend $ 0   $ 0  
Estimated forfeiture rate 0.00%   0.00%  
Weighted average grant date fair value $ 1.15   $ 1.15  
ESPP | Minimum        
Expected term (in years) 6 months   6 months  
Volatility 48.00%   48.00%  
Risk-free interest rate 2.40%   2.40%  
ESPP | Maximum        
Expected term (in years) 2 years   2 years  
Volatility 64.00%   64.00%  
Risk-free interest rate 2.82%   2.82%