Financial Instruments And Risk Management (Reconciliation Of Unrealized Risk Management Positions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Risk Management and Other Derivative Guarantee, at Fair Value, Net, Start of year	$ (292)
Fair Value of Contracts Realized During the Period	(51)	$ (353)	$ (901)
Risk Management and Other Derivative Guarantee, at Fair Value, Net, End of Period	183	(292)
Unrealized Gain (Loss) on Derivatives	474	(615)	(331)
Commodity Contract [Member]
Derivative [Line Items]
Increase (Decrease) in Derivative Assets and Liabilities	525	$ (262)	$ 570
Other Derivative Contracts [Member]
Derivative [Line Items]
Increase (Decrease) in Derivative Assets and Liabilities	7
Other Derivative Contracts Entered Current Year [Member]
Derivative [Line Items]
Increase (Decrease) in Derivative Assets and Liabilities	$ (6)