Fair Value Measurements (Schedule Of Level 3 Financial Instruments) (Details) - Fuel Swap Contracts [Member] $ in Millions		Dec. 31, 2019 USD ($) $ / gal	Dec. 31, 2018 USD ($) $ / gal	Dec. 31, 2017 USD ($)
Information relating to the significant unobservable inputs of Level 3 financial instruments
Fair value | $		$ 1	$ (4)	$ 3
Discounted Cash Flows [Member] | Minimum [Member]
Information relating to the significant unobservable inputs of Level 3 financial instruments
Forward Unleaded Price per Gallon (in dollars per gallon)	[1]	2.37	2.09
Discounted Cash Flows [Member] | Maximum [Member]
Information relating to the significant unobservable inputs of Level 3 financial instruments
Forward Unleaded Price per Gallon (in dollars per gallon)	[1]	2.80	2.43
Discounted Cash Flows [Member] | Weighted Average [Member]
Information relating to the significant unobservable inputs of Level 3 financial instruments
Forward Unleaded Price per Gallon (in dollars per gallon)	[1]	2.61	2.26

[1] Forward prices per gallon were derived from third-party market data providers. A decrease in the forward price would result in an increase in the fair value liability of the fuel swap contracts.