Property, Plant And Equipment (Net Changes in Capitalized Exploratory Well Costs) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Property, Plant And Equipment [Abstract]
Beginning balance at January 1	$ 175,640	$ 148,500
Additions pending the determination of proved reserves	41,940	51,614
Reclassifications to proved properties based on the determination of proved reserves	(2,214)	(13,370)
Capitalized exploratory well costs charged to expense	(4,521)	(8,360)
Ending balance at September 30	$ 210,845	$ 178,384