Fair Value Measurements (Reconciliations For Change In Net Fair Value Of Derivative Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value Measurements [Abstract]
Balance at beginning of period	$ (52)	$ (83)	$ (195)	$ 3
Included in earnings	42	58	141	(13)
Settlements	(10)		34	(15)
Balance at end of period	(20)	(25)	(20)	(25)
Change in gains (losses) included in earnings relating to derivatives still held as of September 30	$ 32	$ 58	$ 175	$ (28)