Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 63,819	$ 66,627	$ 67,840	$ 64,208
Workers' compensation	11,178	11,510
Finance lease obligations	6,354	7,379
Contract liability	6,343	7,061
Separation benefit plans	9,719	10,122
Deferred compensation plan	5,303	6,180
Gas balancing liability	3,838	3,838
Other liabilities	106,554	112,717
Current portion of other long-term liabilities	18,317	17,376
Other long-term liabilities	$ 88,237	$ 95,341