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Inventories (Details) - USD ($)
$ in Thousands
12 Months Ended
May 31, 2018
May 31, 2017
Inventories    
Raw materials $ 18,678 $ 17,563
Work in process 10,808 12,602
Finished goods 19,430 24,341
Total 48,916 54,506
Inventory valuation reserve 6,100 7,300
Japanese inventory distributor    
Inventory [Line Items]    
Inventory write-off 700  
Acculis inventory    
Inventory [Line Items]    
Inventory write-off $ 1,600 $ 2,400