Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 23,843	$ 23,356	$ 63,819	$ 66,627
Workers' compensation	11,735	10,164
Finance lease obligations	2,149	3,216
Contract liability	3,462	4,172
Separation benefit plans	3,859	4,201
Gas balancing liability	4,238	3,997
Other long-term liability	1,320	1,321
Other liabilities	50,606	50,427
Current portion of other long-term liabilities	10,418	11,168
Total other long-term liabilities	$ 40,188	$ 39,259