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Note 8 - Income Taxes - Summary of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Net operating loss and tax credit carryforwards | $ 24,239 | $ 25,745
Joint venture and other investments | (27) | (22)
Accrued retirement benefits | 3,055 | 2,483
Property net book value | 2,266 | 1,716
Deferred revenue | 666 | 546
Stock compensation | 16 | 13
Reserves and other | 189 | (115)
Total deferred tax assets | 30,404 | 30,366
Valuation allowance | (30,404) | (30,366)
Net deferred tax assets |  |

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