Leases - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) aircraft airport lease transaction
Lessee, Lease, Description [Line Items]
Leases that have not yet commenced | $	$ 734
Aircraft sale leaseback
Lessee, Lease, Description [Line Items]
Sale leaseback transactions | $	$ 2,800
Number of aircraft in sale-leaseback transactions	85
Number of sale-leaseback transactions that did not qualify as sale | transaction	74
Number of sale-leaseback transactions that qualified as sale | transaction	11
Aircraft sale leaseback | A321-200
Lessee, Lease, Description [Line Items]
Number of aircraft in sale-leaseback transactions	25
Aircraft sale leaseback | A220-100
Lessee, Lease, Description [Line Items]
Number of aircraft in sale-leaseback transactions	25
Aircraft sale leaseback | CRJ-900
Lessee, Lease, Description [Line Items]
Number of aircraft in sale-leaseback transactions	23
Aircraft sale leaseback | 737-900ER
Lessee, Lease, Description [Line Items]
Number of aircraft in sale-leaseback transactions	10
Aircraft sale leaseback | A330-900
Lessee, Lease, Description [Line Items]
Number of aircraft in sale-leaseback transactions	2
Minimum
Lessee, Lease, Description [Line Items]
Leases that have not yet commenced, term of contract	7 years
Maximum
Lessee, Lease, Description [Line Items]
Leases that have not yet commenced, term of contract	12 years
Aircraft
Lessee, Lease, Description [Line Items]
Number of leases | lease	353
Number of finance leases | lease	145
Number of operating leases | lease	208
Lease component of purchase agreements, number of aircraft	125
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Remaining term of finance leases	1 month
Remaining term of operating leases	1 month
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Remaining term of finance leases	15 years
Remaining term of operating leases	15 years
Airport Facilities
Lessee, Lease, Description [Line Items]
Number of airports with facility space under lease | airport	300
Airport Facilities | Minimum
Lessee, Lease, Description [Line Items]
Remaining term of operating leases	1 month
Airport Facilities | Maximum
Lessee, Lease, Description [Line Items]
Remaining term of operating leases	30 years
Other Ground Property and Equipment | Minimum
Lessee, Lease, Description [Line Items]
Remaining term of finance leases	1 month
Remaining term of operating leases	1 month
Other Ground Property and Equipment | Maximum
Lessee, Lease, Description [Line Items]
Remaining term of finance leases	9 years
Remaining term of operating leases	9 years