Note 5 - Income Taxes - Net Deferred Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Reserves and accrued liabilities	$ 12,050	$ 9,050
Other	1,768	2,464
Deferred Tax Assets, Gross, Total	13,818	11,514
Deferred tax liabilities:
Depreciation	131,650	130,686
Prepaid expenses	3,266	2,850
Deferred Tax Liabilities, Gross, Total	134,916	133,536
Net deferred tax liability	$ 121,098	$ 122,022