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Note 7 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Deferred tax assets:    
Insurance accruals $ 34,788 $ 33,522
Allowance for doubtful accounts 2,347 2,335
Compensation accrual 10,443 13,991
Deferred compensation accrual 26,062 24,687
Federal benefit of state uncertain tax positions 14,085 12,751
Other 3,853 4,036
Total gross deferred tax assets 91,578 91,322
Valuation allowance (552)
Total deferred tax assets, net of valuation allowance 91,578 90,770
Deferred tax liabilities:    
Plant and equipment, principally due to differences in depreciation 831,555 786,840
Prepaid permits and insurance, principally due to expensing for income tax purposes 37,119 33,064
Other 13,538 11,086
Total gross deferred tax liabilities 882,212 830,990
Net deferred tax liability $ 790,634 $ 740,220