Property, Plant And Equipment (Net Changes in Capitalized Exploratory Well Costs) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Property, Plant And Equipment [Abstract]
Beginning balance at January 1	$ 148,500	$ 130,514
Additions pending the determination of proved reserves	48,764	800
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 June 30	$ 175,534	$ 128,109