Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020
Cash Flow Hedging
Derivative Instruments, (Gain) Loss
(Gain) Loss recognized in AOCI on derivatives, net of tax	$ (463)	$ 106
Cash Flow Hedging | Fuel derivative contracts
Derivative Instruments, (Gain) Loss
(Gain) Loss recognized in AOCI on derivatives, net of tax	(461)	80
Cash Flow Hedging | Interest rate derivatives
Derivative Instruments, (Gain) Loss
(Gain) Loss recognized in AOCI on derivatives, net of tax	$ (2)	$ 26
Not Designated as Hedging Instrument
Derivative Instruments, (Gain) Loss
(Gain) Loss recognized in income on derivatives			$ (13)	$ 29
Not Designated as Hedging Instrument | Fuel derivative contracts
Derivative Instruments, (Gain) Loss
(Gain) Loss recognized in income on derivatives			(13)	1
Not Designated as Hedging Instrument | Interest rate derivatives
Derivative Instruments, (Gain) Loss
(Gain) Loss recognized in income on derivatives			$ 0	$ 28