Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Sep. 01, 2020	Aug. 31, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 26,372	$ 23,356	$ 24,922	$ 24,591	$ 38,984	$ 66,627
Workers' compensation	11,311	10,164
Finance lease obligations	0	3,216
Contract liability	2,077	4,172
Separation benefit plans	2,675	4,201
Gas balancing liability	4,238	3,997
Other long-term liability	1,323	1,321
Other liabilities	47,996	50,427
Current portion of other long-term liabilities	6,522	11,168
Total other long-term liabilities	$ 41,474	$ 39,259