Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 175,534	$ 128,109	$ 148,500	$ 130,514
No. of Wells	12	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 | $	$ 57,900	$ 63,617
No. of Wells	3	5
No. of Projects	3	5
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 53,023
No. of Wells	3
No. of Projects	3
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $		$ 31,627
No. of Wells		2
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 64,611	$ 32,865
No. of Wells	6	4