Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 210,845	$ 178,384	$ 175,640	$ 148,500
No. of Wells	10	13
No. of Projects	6	6
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 46,813	$ 41,609
No. of Wells	1	3
No. of Projects	1	2
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 41,051	$ 8,430
No. of Wells	3	2
No. of Projects	2	2
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 5,208	$ 43,197
No. of Wells	1	1
No. of Projects	1	1
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 117,773	$ 85,148
No. of Wells	5	7
No. of Projects	2	1