Property, Plant And Equipment (Net Changes in Capitalized Exploratory Well Costs) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Property, Plant And Equipment [Abstract]
Beginning balance at January 1	$ 175,640,000	$ 148,500,000
Additions pending the determination of proved reserves	30,731,000	48,764,000
Reclassifications to proved properties based on the determination of proved reserves		(13,370,000)
Capitalized exploratory well costs charged to expense	0	(8,360,000)
Ending balance at June 30	$ 206,371,000	$ 175,534,000