Note 6 - Income Taxes - Net Deferred Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Reserves and accrued liabilities	$ 9,050	$ 7,986
Other	2,464	2,313
	11,514	10,299
Deferred tax liabilities:
Depreciation	130,686	113,773
Prepaid expenses	2,850	2,503
	133,536	116,276
Net deferred tax liability	$ 122,022	$ 105,977