Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 175,640	$ 148,500	$ 130,514	$ 120,455
No. of Wells	13	12	13
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 | $	$ 41,480	$ 20,481	$ 66,032
No. of Wells	3	1	7
No. of Projects	2	1	6
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 5,812	$ 63,527
No. of Wells	1	5
No. of Projects	1	5
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 43,200		$ 57,876
No. of Wells	2		3
No. of Projects	2
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 85,148	$ 64,492	$ 6,606
No. of Wells	7	6	3
No. of Projects	1