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
	Mar. 31, 2017	Mar. 31, 2016
Fair Value Measurements [Abstract]
Balance at beginning of period	$ (195)	$ 3
Included in earnings	13	(34)
Settlements	14	(4)
Balance at end of period	(168)	(35)
Change in gains (losses) included in earnings relating to derivatives still held as of March 31	$ 27	$ (38)