Note 7 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Insurance accruals	$ 27,487	$ 24,304
Allowance for doubtful accounts	8,886	9,270
Compensation accrual	22,061	10,809
CARES Act payroll tax deferral	9,955	19,931
Deferred compensation accrual	28,937	25,702
Federal benefit of state uncertain tax positions	12,870	10,312
Lease liabilities	43,850	32,625
State NOL carry-forward	7,303	8,460
Other	5,549	4,237
Total gross deferred tax assets	166,898	145,650
Valuation allowance	(7,303)	(8,460)
Total deferred tax assets, net of valuation allowance	159,595	137,190
Deferred tax liabilities:
Plant and equipment, principally due to differences in depreciation	816,744	748,883
Prepaid permits and insurance, principally due to expensing for income tax purposes	44,132	42,126
Lease right-of-use assets	44,161	32,952
Other	0	5,251
Total gross deferred tax liabilities	905,037	829,212
Net deferred tax liability	$ 745,442	$ 692,022