Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at statutory rate	$ 49,150	$ 54,621	$ 71,118
Percentage depletion allowance	(17,743)	(17,725)	(31,255)
State taxes, net of effect of federal taxes	(12,769)	4,480	7,002
Change in valuation allowance	(24,206)	(79,961)	(410,983)
Impact of Tax Cuts and Jobs Act of 2017	0	(17,645)	332,345
Other, net	5,816	3,754	(3,482)
Provision for (benefit from) income taxes	$ 248	$ (52,476)	$ (35,255)

Amount of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operations, attributable to Tax Cuts and Jobs Act.

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