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Marketable Securities (Details 1) - USD ($)
$ in Thousands
May 31, 2018
May 31, 2017
Amortized cost and fair value of marketable securities by contractual maturity    
Amortized cost, Due in one year or less $ 0  
Amortized cost, Due after one through five years 0  
Amortized cost, Due after five through twenty years 1,350  
Amortized Cost 1,350 $ 1,350
Fair Value, Due in one year or less 0  
Fair Value, Due after one through five years 0  
Fair Value, Due after five through twenty years 1,317  
Fair Value, Total $ 1,317