Leases - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($) aircraft
Lessee, Lease, Description [Line Items]
Additional leases that have not yet commenced | $	$ 570
Maximum
Lessee, Lease, Description [Line Items]
Lease terms of additional leases that have not yet commenced (up to)	34 years
Mainline Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft subject to operating leases	121
Number of aircraft subject to finance leases	26
Mainline Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Initial term of operating leases	1 month
Mainline Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Initial term of operating leases	10 years
Regional Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft subject to operating leases	339
Number of aircraft subject to finance leases	47
Non-Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Initial term of operating leases	1 month
Non-Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Initial term of operating leases	34 years