Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 23,172	$ 23,356	$ 64,248	$ 66,627
Workers' compensation	11,730	10,164
Finance lease obligations	0	3,216
Contract liability	2,724	4,172
Separation benefit plans	3,237	4,201
Gas balancing liability	4,238	3,997
Other long-term liability	1,321	1,321
Other liabilities	46,422	50,427
Current portion of other long-term liabilities	7,551	11,168
Total other long-term liabilities	$ 38,871	$ 39,259