Note 7 - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Insurance accruals	$ 33,522	$ 30,565
Allowance for doubtful accounts	2,335	2,291
Compensation accrual	13,991	15,260
Deferred compensation accrual	24,687	25,042
Federal benefit of state uncertain tax positions	$ 12,751	12,268
Capital loss carry-forward		1,443
Other	$ 4,036	3,954
Total gross deferred tax assets	91,322	90,823
Valuation allowance	(552)	(1,994)
Total deferred tax assets, net of valuation allowance	90,770	88,829
Deferred tax liabilities:
Plant and equipment, principally due to differences in depreciation	786,840	707,071
Prepaid permits and insurance, principally due to expensing for income tax purposes	33,064	30,012
Other	11,086	11,539
Total gross deferred tax liabilities	830,990	748,622
Net deferred tax liability	$ 740,220	$ 659,793