Note 6 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Insurance accruals	$ 104,720	$ 54,047
Allowance for doubtful accounts	10,766	10,230
Compensation accrual	8,003	30,492
Deferred compensation accrual	36,309	28,249
Federal benefit of state uncertain tax positions	17,491	16,280
Lease liabilities	82,048	71,732
State NOL carry-forward	5,478	6,765
Other	7,673	7,361
Total gross deferred tax assets	272,488	225,156
Valuation allowance	(5,478)	(6,765)
Total deferred tax assets, net of valuation allowance	267,010	218,391
Deferred tax liabilities:
Plant and equipment, principally due to differences in depreciation	1,057,922	1,011,963
Prepaid permits and insurance, principally due to expensing for income tax purposes	63,880	55,132
Lease right-of-use assets	81,416	71,827
Total gross deferred tax liabilities	1,203,218	1,138,922
Net deferred tax liability	$ 936,208	$ 920,531