Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in AOCI on derivatives, net of tax	$ (186)	$ (9)	$ (279)	$ 107
Cash Flow Hedging | Fuel derivatives
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in AOCI on derivatives, net of tax	(192)	(9)	(275)	75
Cash Flow Hedging | Interest rate derivatives
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in AOCI on derivatives, net of tax	6	0	(4)	32
Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives	(12)	6	(16)	30
Not Designated as Hedging Instrument | Fuel derivatives | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives	(12)	1	(16)	1
Not Designated as Hedging Instrument | Interest rate derivatives | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives	$ 0	$ 5	$ 0	$ 29