Note 7 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Insurance accruals	$ 27,180	$ 34,889
Allowance for doubtful accounts	8,052	7,649
Compensation accrual	4,925	10,461
Deferred compensation accrual	24,521	20,396
Federal benefit of state uncertain tax positions	9,867	10,364
Lease liabilities	30,251	0
State NOL carry-forward	7,495	6,041
Other	6,357	4,626
Total gross deferred tax assets	118,648	94,426
Valuation allowance	(7,495)	(6,041)
Total deferred tax assets, net of valuation allowance	111,153	88,385
Deferred tax liabilities:
Plant and equipment, principally due to differences in depreciation	729,016	696,913
Prepaid permits and insurance, principally due to expensing for income tax purposes	39,285	33,594
Lease right-of-use assets	30,014	0
Other	11,916	1,339
Total gross deferred tax liabilities	810,231	731,846
Net deferred tax liability	$ 699,078	$ 643,461