Note 6 - Income Taxes - Deferred Tax Assets And Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net operating loss	$ 60,268	$ 49,626	$ 45,831
Stock option expense	6,454	5,063	1,155
Property and equipment	(11,941)	(10,372)	(9,688)
Other	1,194	966	552
Less: Valuation allowance	(55,975)	(45,283)	(37,850)
Net deferred income tax assets	$ 0	$ 0	$ 0

Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, without jurisdictional netting.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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