Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 176,263	$ 166,423	$ 175,640	$ 148,500
No. of Wells	12	11
No. of Projects	5	5
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 13,642	$ 48,202
No. of Wells	2	2
No. of Projects	1	2
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 27,757	$ 53,729
No. of Wells	1	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 | $	$ 49,642
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 | $	$ 85,222	$ 64,492
No. of Wells	7	6
No. of Projects	1