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Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Derivative [Line Items]        
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax $ 3 $ 2    
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax (11) 0    
Accumulated Other Comprehensive Income (Loss), Net of Tax (6) (1) $ 2 $ (3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax 1 0    
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax (4) 0    
Fuel Derivatives [Member]        
Derivative [Line Items]        
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax 3 2    
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax (11) 0    
Accumulated Other Comprehensive Income (Loss), Net of Tax $ (6) $ (1) $ 2 $ (3)