Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash Flow Hedging
Derivative Instruments, (Gain) Loss
(Gain) loss recognized in AOCI on derivatives (effective portion)	$ 106	$ 119
Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, (Gain) Loss
(Gain) loss recognized in income on derivatives	29	0
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, (Gain) Loss
(Gain) loss recognized in AOCI on derivatives (effective portion)	80	90
Fuel derivatives | Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, (Gain) Loss
(Gain) loss recognized in income on derivatives	1	0
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, (Gain) Loss
(Gain) loss recognized in AOCI on derivatives (effective portion)	26	29
Interest rate derivatives | Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, (Gain) Loss
(Gain) loss recognized in income on derivatives	$ 28	$ 0