Property, Plant And Equipment (Net Changes in Capitalized Exploratory Well Costs) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property, Plant And Equipment [Abstract]
Beginning balance at January 1	$ 175,640,000	$ 148,500,000
Additions pending the determination of proved reserves	623,000	39,653,000
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 March 31	$ 176,263,000	$ 166,423,000