Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Taxes
Income tax provision at statutory rate	$ (72,371)	$ 49,150	$ 54,621
Percentage depletion allowance	(7,763)	(17,743)	(17,725)
State taxes, net of effect of federal taxes	(3,298)	(12,769)	4,480
Change in valuation allowance	76,524	(24,206)	(79,961)
Other, net	6,901	5,816	3,754
Provision for (benefit from) income taxes	$ (7)	$ 248	$ (52,476)

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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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