Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 62,838		$ 69,444	$ 70,049	$ 70,170
Capital lease obligations	13,321		15,224
Workers' compensation	12,963		13,340
Contract liability	11,331		0
Separation benefit plans	7,607		6,524
Deferred compensation plan	5,621		5,390
Gas balancing liability	3,283		3,283
Other liabilities	116,964		113,205
Current portion of other long-term liabilities	14,036	$ 15,750	13,002
Other long-term liabilities	$ 102,928	$ 109,940	$ 100,203