Fair Value Measurements (Schedule Of Level 3 Financial Instruments) (Details) - Fuel Swap Contracts [Member] $ in Millions		Mar. 31, 2021 USD ($) $ / gal	Dec. 31, 2020 USD ($) $ / gal	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Information relating to the significant unobservable inputs of Level 3 financial instruments
Fair value | $		$ 6	$ 3	$ (8)	$ 1
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]	2.64	2.20
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]	2.98	2.58
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)		2.83	2.44

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