Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 67,840	$ 64,208	$ 63,763	$ 69,444
Workers' compensation	12,226	12,738
Finance lease obligations	10,395	11,380
Contract liability	9,173	9,881
Separation benefit plans	9,291	8,814
Deferred compensation plan	5,845	5,132
Gas balancing liability	3,372	3,331
Other liabilities	118,142	115,484
Current portion of other long-term liabilities	14,310	14,250
Other long-term liabilities	$ 103,832	$ 101,234