Note 7 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Insurance accruals	$ 27,700	$ 34,788
Allowance for doubtful accounts	6,605	2,347
Compensation accrual	3,661	10,443
Deferred compensation accrual	17,620	26,062
Federal benefit of state uncertain tax positions	8,681	14,085
State NOL carry-forward	4,944	3,444
Other	3,134	3,853
Total gross deferred tax assets	72,345	95,022
Valuation allowance	(4,944)	(3,444)
Total deferred tax assets, net of valuation allowance	67,401	91,578
Deferred tax liabilities:
Plant and equipment, principally due to differences in depreciation	566,396	831,555
Prepaid permits and insurance, principally due to expensing for income tax purposes	28,089	37,119
Other	14,786	13,538
Total gross deferred tax liabilities	609,271	882,212
Net deferred tax liability	$ 541,870	$ 790,634