Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 206,371	$ 175,534	$ 175,640	$ 148,500
No. of Wells	13	12
No. of Projects	6	6
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 34,779	$ 57,900
No. of Wells	2	3
No. of Projects	2	3
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 35,934	$ 53,023
No. of Wells	2	3
No. of Projects	1	3
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 50,272
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 | $	$ 85,386	$ 64,611
No. of Wells	7	6
No. of Projects	1