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Note 7 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets:    
Insurance accruals $ 34,889 $ 27,700
Allowance for doubtful accounts 7,649 6,605
Compensation accrual 10,461 3,661
Deferred compensation accrual 20,396 17,620
Federal benefit of state uncertain tax positions 10,364 8,681
State NOL carry-forward 6,041 4,944
Other 4,626 3,134
Total gross deferred tax assets 94,426 72,345
Valuation allowance (6,041) (4,944)
Total deferred tax assets, net of valuation allowance 88,385 67,401
Deferred tax liabilities:    
Plant and equipment, principally due to differences in depreciation 696,913 566,396
Prepaid permits and insurance, principally due to expensing for income tax purposes 33,594 28,089
Other 1,339 14,786
Total gross deferred tax liabilities 731,846 609,271
Net deferred tax liability $ 643,461 $ 541,870