Segment Information (Reconciliation Statement Of Segment Income from Operations To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting [Abstract]
Net income	$ 72,741	$ 59,985
Provision for (benefit from) income taxes	70	(544)
Interest expense, net	2,289	4,122
Depreciation, depletion and amortization	25,273	29,703
Accretion on asset retirement obligations	5,137	6,992
Amortization of sales contracts, net	65	3,051
Non-service related pension and postretirement benefit costs	1,766	1,303
Reorganization items, net	(87)	301
Adjusted EBITDA	$ 107,254	$ 104,913