Segment Information (Reconciliation Statement Of Segment Income from Operations To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting [Abstract]
Income before income taxes	$ 59,441	$ 52,508
Interest expense, net	4,122	8,898
Depreciation, depletion and amortization	29,703	31,921
Accretion on asset retirement obligations	6,992	7,623
Amortization of sales contracts, net	3,051	14,690
Net loss resulting from early retirement of debt and debt restructuring	0	2,030
Non-service related pension and postretirement benefit costs	1,303	721
Reorganization items, net	301	2,828
Adjusted EBITDA	$ 104,913	$ 121,219