Long-Term Debt And Other Long-Term Liabilities (Other Long-Term Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Asset retirement obligation (ARO) liability	$ 63,763		$ 69,444	$ 70,043	$ 70,170
Capital lease obligations	14,277		15,224
Workers' compensation	13,049		13,340
Contract liability	11,942		0
Separation benefit plans	7,087		6,524
Deferred compensation plan	5,472		5,390
Gas balancing liability	3,283		3,283
Other liabilities	118,873		113,205
Current portion of other long-term liabilities	14,587	$ 15,750	13,002
Other long-term liabilities	$ 104,286	$ 109,940	$ 100,203