Comprehensive Income (Loss) (Schedule Of Reclassifications Out Of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	$ (27)	$ (37)	$ 242
Net Income (Loss)	128	(41)	510
Unrealized Gains (Losses) on Derivatives [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net losses on derivatives	5	4	(6)
Provision for income taxes		(1)	2
Net Income (Loss)	4	3	(4)
Unrealized Gains (Losses) on Derivatives [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Fuel Swap Contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains (losses) on derivatives:		3	3
Unrealized Gains (Losses) on Derivatives [Member] | Amount Reclassified from Accumulated Other Comprehensive Income [Member] | Interest Rate Swap Contracts [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains (losses) on derivatives:	$ 5	$ 1	$ (8)