Leases and Aircraft Maintenance Deposits Leases and Aircraft Maintenance Deposits - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2020 aircraft	Dec. 31, 2019 USD ($) a aircraft_engine aircraft	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) aircraft	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Total rental expense		$ 345,000	$ 312,000	$ 309,800
Number of spare engines capitalized | aircraft		14
Rental expense charged to operations for aircraft and engine operating leases		$ 182,609	177,641	205,852
Maintenance reserves, supplemental rent		500	1,300	400
Aircraft maintenance deposits		170,600
Aircraft maintenance deposits, current		102,906	106,901
Long-term aircraft maintenance deposits		67,682	138,738
Finance lease, interest expense		674
Finance lease liability		$ 46,081
Area of land | a		8.5
Land		$ 41,000
Operating lease, right-of-use asset		$ 1,369,555	0
Estimated time to complete construction		2 years
Aircraft Supplemental Rent
Lessee, Lease, Description [Line Items]
Supplemental rent expense		$ 4,800	$ 3,400	$ 8,400
Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Operating lease, right-of-use asset					$ 1,000,000
Restatement Adjustment | Accounting Standards Update 2016-02
Lessee, Lease, Description [Line Items]
Retained earnings impact of Topic 842 adoption						$ 5,500
A320 Family
Lessee, Lease, Description [Line Items]
Number of aircraft held | aircraft		145
Number of aircraft purchased | aircraft		4
Consideration transferred		$ 141,300
Airbus A320
Lessee, Lease, Description [Line Items]
Finance lease, interest expense		$ 300
Airbus A319
Lessee, Lease, Description [Line Items]
Number of aircraft under finance lease | aircraft		2
Number of aircraft to be purchased | aircraft		2
Aircraft
Lessee, Lease, Description [Line Items]
Number of previously leased aircraft purchased off lease | aircraft		5
Finance lease liability		$ 44,100
Finance lease, right-of-use asset		$ 48,400
Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft		52
Spare engines
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft_engine		9
Number of spare engines purchased | aircraft_engine		4
Other leased equipment and property
Lessee, Lease, Description [Line Items]
Area of land | a		2.6
Noncancellable short-term operating leases commitments		$ 600
Land
Lessee, Lease, Description [Line Items]
Operating lease, right-of-use asset		14,600
Operating lease rIght-of-use liability		9,300
Minimum
Lessee, Lease, Description [Line Items]
Development in process, estimated total costs		$ 200,000
Minimum | Leased Computer And Office Equipment
Lessee, Lease, Description [Line Items]
Finance lease contract term		3 years
Minimum | Aircraft
Lessee, Lease, Description [Line Items]
Operating leases, term		8 years
Maximum
Lessee, Lease, Description [Line Items]
Development in process, estimated total costs		$ 250,000
Maximum | Leased Computer And Office Equipment
Lessee, Lease, Description [Line Items]
Finance lease contract term		5 years
Maximum | Aircraft
Lessee, Lease, Description [Line Items]
Operating leases, term		18 years
Maximum | Other leased equipment and property
Lessee, Lease, Description [Line Items]
Operating leases, term		99 years
2019
Lessee, Lease, Description [Line Items]
Number of sale and leaseback transactions | aircraft		6
2019 | Aircraft
Lessee, Lease, Description [Line Items]
Number of aircrafts delivered under secured debt arrangements | aircraft		4
2019 | Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of leased assets financed under operating leases | aircraft		7
Subsequent Event | Airbus A319
Lessee, Lease, Description [Line Items]
Number of aircraft purchased | aircraft	2
Spare engines
Lessee, Lease, Description [Line Items]
Number of capitalized aircraft or engine purchased off lease | aircraft		3
Number of previously leased aircraft purchased off lease | aircraft		2
Spare engines | Minimum
Lessee, Lease, Description [Line Items]
Operating leases, term		2 years
Spare engines | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, term		14 years
Aircraft
Lessee, Lease, Description [Line Items]
Number of capitalized aircraft or engine purchased off lease | aircraft		27
Number of previously leased aircraft purchased off lease | aircraft				1
Aircraft | A320 Family
Lessee, Lease, Description [Line Items]
Number of aircraft capitalized | aircraft		91
Aircraft | Minimum
Lessee, Lease, Description [Line Items]
Operating leases, term		8 years
Aircraft | Maximum
Lessee, Lease, Description [Line Items]
Operating leases, term		18 years