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Income Taxes (Details 3) - USD ($)
$ in Thousands
May 31, 2016
May 31, 2015
Deferred tax assets    
Net operating loss carryforward $ 52,593 $ 52,025
Stock-based compensation 4,135 4,468
Federal and state R&D tax credit carryforward 2,145 1,646
Inventories 4,535 2,808
Expenses incurred not currently deductible 3,018 2,107
Deferred revenue 339 114
Gross deferred tax asset 66,765 63,168
Deferred tax liabilities    
Excess tax over book depreciation and amortization 46,240 42,988
Gross deferred Liability 46,240 42,988
Valuation Allowance (42,209) (1,791)
Net deferred tax liability $ (21,684)  
Net deferred tax asset (liability)   $ 18,389