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Income Taxes - Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Deferred Tax Assets:    
Accrued liabilities $ 349 $ 550
Allowance for doubtful accounts 22 152
Derivative Contracts 684 1,273
Alternative minimum tax credits 9,919 6,612
Net operating loss carry-forwards 528 586
Percentage depletion carry-forwards 727 3,025
Total deferred tax assets 12,229 12,198
Deferred Tax Liabilities:    
Basis differences relating to managed partnerships 3,193 6,211
Depletion and depreciation 33,998 43,487
Total deferred tax liabilities 37,191 49,698
Net deferred tax liabilities $ 24,962 $ 37,500