Property, Plant And Equipment (Net Changes in Capitalized Exploratory Well Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant And Equipment [Abstract]
Beginning balance at January 1	$ 148,500	$ 130,514	$ 120,455
Additions pending the determination of proved reserves	51,488	17,986	64,578
Reclassifications to proved properties based on the determination of proved reserves	(15,988)
Capitalized exploratory well costs charged to expense	(8,360)		(54,519)
Ending balance at December 31	$ 175,640	$ 148,500	$ 130,514