Income Taxes (Income Tax Expense (Benefit)) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at statutory rate	$ 8,387	$ 13,118	$ 20,870	$ 31,496
Percentage depletion allowance	(2,488)	(5,692)	(7,095)	(12,731)
State taxes, net of effect of federal taxes	583	408	1,337	891
Change in valuation allowance	(7,317)	(8,316)	(17,956)	(20,218)
Other, net	(2,531)	801	(1,066)	1,721
Provision for (benefit from) income taxes	(3,366)	$ 319	(3,910)	$ 1,159
Reversal of reserve	$ 3,300		$ 3,300

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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