Income Taxes (Reconciliation Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax expense (benefit) computed by applying the statutory rate		$ 21,059	$ (72,386)	$ (582,508)
State income tax expense (benefit), net of federal benefit		1,655	(5,687)	(45,768)
Deferred tax liability revaluation	[1]	(81,307)	0	0
Restricted stock shortfall		1,867	5,465	0
Statutory depletion and other		(952)	1,414	1,328
Income tax benefit		$ (57,678)	$ (71,194)	$ (626,948)

[1] In 2017, the revaluation from the Tax Act.