Leases Narrative (Details) $ in Thousands	Mar. 31, 2019 USD ($) aircraft_engine aircraft	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Lease Not Yet Commenced, Amount	$ 200
Lessee, Operating Lease, Lease Not Yet Commenced, Amount, Year Two	0
Stockholders' Equity Attributable to Parent	$ 1,977,656		$ 1,928,504	$ 1,719,805	$ 1,762,574
Minimum
Lessee, Lease, Description [Line Items]
Finance leases, term	3 years
Maximum
Lessee, Lease, Description [Line Items]
Finance leases, term	5 years
Spare Engines
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft_engine	11
Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft	50
Fixed Maintenance Reserve Payments, Aircraft and Related Flight Equipment
Lessee, Lease, Description [Line Items]
Committed expenditures, remainder of 2019	$ 4,300
Committed expenditures, 2020	5,600
Committed expenditures, 2021	5,700
Committed expenditures, 2022	4,900
Committed expenditures, 2023	4,100
Committed expenditures, 2024 and beyond	$ 8,800
Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft financed under operating leases with a variable rate | aircraft	1
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Operating leases, term	8 years
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, term	18 years
Other Leased Equipment and Property | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, term	30 years
Retained Earnings [Member]
Lessee, Lease, Description [Line Items]
Stockholders' Equity Attributable to Parent	$ 1,676,008		$ 1,625,481	$ 1,424,810	$ 1,469,732
Accounting Standards Update 2016-02 | Restatement Adjustment [Member]
Lessee, Lease, Description [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets		$ (5,500)
Accounting Standards Update 2016-02 | Restatement Adjustment [Member] | Retained Earnings [Member]
Lessee, Lease, Description [Line Items]
Stockholders' Equity Attributable to Parent	$ (5,549)