Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 229,084	$ 175,640	$ 148,500	$ 130,514
No. of Wells	10	13	12
No. of Projects	6	6	6
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 61,096	$ 41,480	$ 20,481
No. of Wells	1	3	1
No. of Projects	1	2	1
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 40,523	$ 5,812	$ 63,527
No. of Wells	3	1	5
No. of Projects	2	1	5
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $		$ 43,200
No. of Wells		2
No. of Projects		2
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 127,465	$ 85,148	$ 64,492
No. of Wells	6	7	6
No. of Projects	3	1