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Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
(Gain) loss recognized in AOCI on derivatives $ 112 $ (122) $ 126 $ (430)
Fuel derivatives | Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
(Gain) loss recognized in AOCI on derivatives 95 (122) 83 (428)
Interest rate derivatives | Cash Flow Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
(Gain) loss recognized in AOCI on derivatives $ 17 $ 0 43 (2)
Interest rate derivatives | Interest rate derivatives | Not Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain recognized in income on derivatives     $ 0 $ (2)