Operating Leases - Aircraft (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) aircraft aircraftEngine	Sep. 30, 2020 USD ($) aircraftEngine aircraft	Sep. 30, 2021 USD ($) aircraftEngine aircraft	Sep. 30, 2020 USD ($) aircraft aircraftEngine	Dec. 31, 2021 aircraft	Jun. 30, 2021 aircraft	May 31, 2021 aircraft	Mar. 31, 2020 USD ($) amendment aircraft	Feb. 28, 2020
Lessee, Lease, Description [Line Items]
Gains recognized on sale-leaseback transactions | $			$ 55	$ 37
Number of amendments | amendment								2
Number of aircraft renewed								2
Length of contract								5 years	3 years
Return of previously unrecoverable maintenance expense | $								$ 17
Number of aircraft terminated							6
Aircraft
Lessee, Lease, Description [Line Items]
Number of leases	112		112
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Remaining lease terms	3 months		3 months
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Remaining lease terms	12 years		12 years
Aircraft Engine
Lessee, Lease, Description [Line Items]
Number of leases | aircraftEngine	17		17
Aircraft Engine | Aircraft Sale Leaseback
Lessee, Lease, Description [Line Items]
Number of sale-leaseback transactions | aircraftEngine	0	1	1	1
Aircraft Engine | Minimum
Lessee, Lease, Description [Line Items]
Remaining lease terms	4 months		4 months
Aircraft Engine | Maximum
Lessee, Lease, Description [Line Items]
Remaining lease terms	11 years		11 years
Aircraft and Aircraft Engines
Lessee, Lease, Description [Line Items]
Gains recognized on sale-leaseback transactions | $	$ 19	$ 18	$ 55	$ 37
Acquired aircraft and engines through operating leases | $			$ 480	$ 218
A-320 | Forecast
Lessee, Lease, Description [Line Items]
Number of aircraft returned					2
A-320 | Aircraft | Aircraft Sale Leaseback
Lessee, Lease, Description [Line Items]
Number of sale-leaseback transactions	5	4	13	7
A-319
Lessee, Lease, Description [Line Items]
Number of aircraft returned	2		2			2
A-319 | Forecast
Lessee, Lease, Description [Line Items]
Number of aircraft terminated					4
A-319, To Be Returned Early
Lessee, Lease, Description [Line Items]
Charge included as a component of rent expense | $	$ (1)		$ (10)