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Income Taxes (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Income Taxes    
Income tax provision at statutory rate $ (5,689) $ 15,290
Percentage depletion allowance 6,168 (4,307)
State taxes, net of effect of federal taxes 82 1,012
Change in valuation allowance 850 (12,513)
Current expense associated with uncertain tax positions (2,031) 593
AMT sequestration refund (1,171)  
Other, net 0 (5)
Provision for (benefit from) income taxes $ (1,791) $ 70