Leases - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) lease numberOfAircraft airport	Dec. 31, 2017 USD ($)	Dec. 31, 2018	Jan. 01, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Remaining term of finance leases	5 years		7 years
Remaining term of operating leases	12 years		12 years
Operating lease expense | $		$ 1,600
Leases that have not yet commenced | $	$ 888
New lease standard
Lessee, Lease, Description [Line Items]
Transition adjustment to reduce equity upon adoption | $				$ 284
Minimum
Lessee, Lease, Description [Line Items]
Leases that have not yet commenced, term of contract	5 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	343
Number of finance leases | lease	130
Number of operating leases | lease	213
Lease component of purchase agreements, number of aircraft | numberOfAircraft	162
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	12 years
Remaining term of operating leases	12 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	31 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	7 years
Remaining term of operating leases	7 years