Income Taxes (Reconciliation of Income Tax Provision/(Benefit)) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Income (loss) before income tax provision (benefit)	$ 5,956	$ (13,721)	$ 23,347	$ (6,398)
Statutory taxes (benefit) at 35.0%	2,085	(4,802)	8,171	(2,239)
Percentage depletion	(1,033)	(7,889)	(5,531)	(12,034)
State and local income taxes	119	(758)	187	(788)
Domestic production deduction	(75)	(628)	(371)	(666)
Non-deductible expenses	(469)	991	168	1,701
Valuation allowances	1,923	1,673	1,692	1,690
Uncertain tax positions	7	190	55	(2,015)
Other, net	68	(366)	193	381
Income tax provision (benefit)	$ 2,625	$ (11,589)	$ 4,564	$ (13,970)
Effective income tax rate	44.10%	84.50%	19.50%	218.30%

Effective Income Tax Rate Reconciliation, Uncertain Tax Positions, Amount

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

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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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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 the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deduction for qualified production activity.

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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 tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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