Income Taxes (Income Tax Expense (Benefit)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at statutory rate	$ 12,483	$ 18,376
Percentage depletion allowance	(4,607)	(7,039)
State taxes, net of effect of federal taxes	754	483
Change in valuation allowance	(10,639)	(11,900)
Other, net	1,465	920
Income Tax Expense (Benefit)	$ (544)	$ 840