Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 227,126	$ 155,652	$ 207,855	$ 155,103
No. of Wells	9	11
No. of Projects	4	4
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 78,016	$ 13,642
No. of Wells	3	2
No. of Projects	2	1
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $		$ 27,757
No. of Wells		1
No. of Projects		1
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 27,270	$ 49,642
No. of Wells	1	2
No. of Projects	1	2
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 121,840	$ 64,611
No. of Wells	5	6
No. of Projects	1