Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 178,384	$ 128,156	$ 148,500	$ 130,514
No. of Wells	13	11
No. of Projects	6	5
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 41,609	$ 10,563
No. of Wells	3	2
No. of Projects	2	2
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 8,430	$ 53,101
No. of Wells	2	3
No. of Projects	2	3
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 43,197	$ 31,627
No. of Wells	1	2
No. of Projects	1
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 85,148	$ 32,865
No. of Wells	7	4
No. of Projects	1