Property, Plant And Equipment (Net Changes in Capitalized Exploratory Well Costs) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Property, Plant And Equipment [Abstract]
Beginning balance at January 1	$ 148,500	$ 130,514
Additions pending the determination of proved reserves	51,614	847
Reclassifications to proved properties based on the determination of proved reserves	(13,370)
Capitalized exploratory well costs charged to expense	(8,360)
Other adjustments		(3,205)
Balance at September 30	$ 178,384	$ 128,156