Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 62,727		$ 69,444	$ 75,485	$ 70,170
Workers' compensation	12,832		13,340
Capital lease obligations	12,355		15,224
Contract liability	10,605		0
Separation benefit plans	8,135		6,524
Deferred compensation plan	5,623		5,390
Gas balancing liability	3,283		3,283
Other liabilities	115,560		113,205
Current portion of other long-term liabilities	14,150	$ 15,750	13,002
Other long-term liabilities	$ 101,410	$ 109,940	$ 100,203