Leases - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) aircraft
Lessee, Lease, Description [Line Items]
Additional leases that have not yet commenced | $	$ 140
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	122
Number of aircraft subject to finance leases	30
Mainline Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Initial term of operating leases	2 months
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	341
Number of aircraft subject to finance leases	76
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