Leases (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) aircraft
Sublease Income	$ 25,000,000
Document Period End Date	Mar. 31, 2019
Sublease income - Remainder of 2019	$ 73,000,000
Sublease income - 2020	78,000,000
Unrecorded Unconditional Purchase Obligation	$ 600,000,000
Lessee, Operating Lease, Lease Not yet Commenced, Description | aircraft	16
Sublease income - 2021	$ 41,000,000
Sublease income - 2022	17,000,000
Sublease income - 2023	$ 7,000,000
B-717-200
Property Subject to or Available for Operating Lease, Number of Units | aircraft	73
Finance Leased Assets, Number of Units | aircraft	2
Property Subject To Or Available For Lease, Number Of Units, Total | aircraft	88
Finance Leases, Contingent Rental Payments Due	$ 0
Owned Assets, Number of Units | aircraft	10
Lease Terms For Owned Aircraft	4 years
Minimum | B-717-200
Sublease Terms	6 months
Minimum | 737 MAX 8
Lessee, Renewal Term	8 years
Minimum | Aircraft
Lessee, Renewal Term	1 year
Lessee Operating and Finance Lease Term of Contract	6 months
Minimum | Airport-Related
Lessee Operating Lease Terms of Contract	2 months
Minimum | Other Property
Lessee Operating and Finance Lease Term of Contract	1 month
Maximum | B-717-200
Sublease Terms	5 years
Maximum | 737 MAX 8
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	9 years
Maximum | Aircraft
Lessee, Renewal Term	5 years
Lessee Operating and Finance Lease Term of Contract	13 years
Maximum | Airport-Related
Lessee Operating Lease Terms of Contract	27 years
Maximum | Other Property
Lessee, Renewal Term	5 years
Lessee Operating and Finance Lease Term of Contract	8 years