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Income Taxes (Details 3) - USD ($)
$ in Thousands
May 31, 2018
May 31, 2017
Deferred tax assets    
Net operating loss carryforward $ 33,880 $ 55,975
Stock-based compensation 2,421 2,653
Federal and state R&D tax credit carryforward 3,644 2,548
Inventories 1,550 2,407
Expenses incurred not currently deductible 1,714 6,522
Deferred revenue 277 1,289
Gross deferred tax asset 43,486 71,394
Deferred tax liabilities    
Excess tax over book depreciation and amortization 34,044 49,158
Gross deferred tax liability 34,044 49,158
Valuation Allowance (26,607) (48,348)
Net deferred tax liability (17,165) $ (26,112)
Non-U.S.    
Deferred tax assets    
Expenses incurred not currently deductible $ 80