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Note 8 - Income Taxes - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Net operating loss and tax credit carryforwards | $ 24,956 | $ 29,545
Joint venture and other investments | (27) | (27)
Accrued retirement benefits | 3,329 | 3,010
Property net book value | 3,357 | 3,300
Deferred revenue | 715 | 697
Reserves and other | 29 | (10)
Total deferred tax assets | 32,359 | 36,515
Valuation allowance | (32,359) | (36,515)
Net deferred tax assets |  |

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- Definition Amount before allocation of valuation allowances of deferred tax asset (liability) attributable to deductible temporary differences from reserves and accruals. + References No definition available. + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards and the tax effect as of the balance sheet date of the amount of future tax deductions arising from all unused tax credit carryforwards. + References No definition available. + Details
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