Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability		$ 64,248	$ 66,627	$ 67,433	$ 64,208
Workers' compensation		12,112	11,510
Finance lease obligations		5,319	7,379
Contract liability		5,625	7,061
Separation benefit plans	[1]	0	10,122
Deferred compensation plan		6,006	6,180
Gas balancing liability		3,823	3,838
Other long-term liability		1,217	0
Other liabilities		98,350	112,717
Current portion of other long-term liabilities		13,628	17,376
Total other long-term liabilities		$ 84,722	$ 95,341

[1] The separation benefit plans are part of the liabilities subject to compromise as of June 30, 2020. For further information, please see Note 2 – Chapter 11 Proceedings, Liquidity, and Ability to Continue as a Going Concern.