Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	May 31, 2014	May 31, 2013
Deferred tax assets
Net operating loss carryforward	$ 48,749	$ 47,516
Stock-based compensation	4,851	4,813
Federal and state R&D tax credit carryforward	1,249	490
Inventories	875us-gaap_DeferredTaxAssetsInventory	1,713us-gaap_DeferredTaxAssetsInventory
State tax credits	0us-gaap_DeferredTaxAssetsStateTaxes	1,326us-gaap_DeferredTaxAssetsStateTaxes
Expenses incurred not currently deductible	1,379	880
Capital loss carryforwards	0	95
Deferred revenue	154us-gaap_DeferredTaxAssetsDeferredIncome	1,147us-gaap_DeferredTaxAssetsDeferredIncome
Gross deferred tax asset	57,257us-gaap_DeferredTaxAssetsGross	57,980us-gaap_DeferredTaxAssetsGross
Deferred tax liabilities
Excess tax over book depreciation and amortization	41,843	39,252
Impairment of long-lived assets	0	0
Gross deferred Liability	41,843us-gaap_DeferredIncomeTaxLiabilities	39,252us-gaap_DeferredIncomeTaxLiabilities
Valuation Allowance	(1,532)	(712)
Net deferred tax asset	$ 13,882us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 18,016us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred tax liabilities impairment losses.

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Amount of deferred tax liability attributable to taxable temporary differences.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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