Property, Plant And Equipment (Aging of Capitalized Exploratory Well Costs) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item	Dec. 31, 2013 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 148,500,000	$ 130,514,000	$ 120,455,000	$ 393,030,000
No. of Wells	12	13	8
No. of Projects	6	6	3
Zero to One Year [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 20,481,000	$ 66,032,000
No. of Wells	1	7
No. of Projects	1	6
One to Two Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 63,527,000		$ 59,330,000
No. of Wells	5		3
No. of Projects	5		1
Two to Three Years [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $		$ 57,876,000	$ 6,606,000
No. of Wells		3	3
Three Years or More [Member]
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 64,492,000	$ 6,606,000	$ 54,519,000
No. of Wells	6	3	2
No. of Projects			2