Leases and Prepaid Maintenance Deposits - Operating Leases (Narrative) (Details) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) aircraft_engine aircraft	Dec. 31, 2017 USD ($) aircraft	Dec. 31, 2016 USD ($) aircraft
Operating Leased Assets [Line Items]
Total rental expense					$ 312,000	$ 309,800	$ 283,900
Rental expense charged to operations for aircraft and engine operating leases	$ 42,820	$ 53,396	$ 52,566	$ 57,070	177,641	205,852	201,675
Maintenance reserves, supplemental rent					1,300	400	2,200
Aircraft maintenance deposits					245,600
Aircraft maintenance deposits, net	175,615				106,901	175,615	87,035
Long-term aircraft maintenance deposits	$ 150,617				$ 138,738	150,617	199,415
Number of Spare Engines Capitalized | aircraft					8
Number of leased engines delivered | aircraft_engine					1
Property, Plant And Equipment, Number Of Engines Sold | aircraft_engine					6
Spare engines
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft_engine					12
A320 Family
Operating Leased Assets [Line Items]
Number of Aircraft Held | aircraft					128
A320 Family | Aircraft
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft					46
Fixed Maintenance Reserve Payments
Operating Leased Assets [Line Items]
Committed expenditures, 2019					$ 5,800
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 Supplemental Rent
Operating Leased Assets [Line Items]
Supplemental rent expense					$ 3,400	$ 8,400	$ 9,000
Minimum
Operating Leased Assets [Line Items]
Lease term					3 years
Maximum
Operating Leased Assets [Line Items]
Lease term					5 years
Aircraft
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft					14	1
Number of aircraft and spare engines financed under operating leases with a variable rate | aircraft					1
Number of aircrafts delivered under secured debt arrangements | aircraft					14
Aircraft | A320 Family
Operating Leased Assets [Line Items]
Number of aircraft capitalized | aircraft					82
Number of previously leased aircraft purchased off lease | aircraft					22
Aircraft | Airbus A319
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft							7
Aircraft | Minimum
Operating Leased Assets [Line Items]
Operating leases, term					8 years
Aircraft | Maximum
Operating Leased Assets [Line Items]
Operating leases, term					18 years
Other leased equipment and property | Maximum
Operating Leased Assets [Line Items]
Operating leases, term					30 years
Spare engines
Operating Leased Assets [Line Items]
Number of spare engines purchased | aircraft_engine					10
Spare engines | Minimum
Operating Leased Assets [Line Items]
Operating leases, term					2 years
Spare engines | Maximum
Operating Leased Assets [Line Items]
Operating leases, term					14 years
Third Party Lessor
Operating Leased Assets [Line Items]
Number of direct lease aircraft from a third-party lessor | aircraft					2
Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft					1
Proceeds from Sale of Other Property, Plant, and Equipment					$ 11,400
Gain (Loss) on Disposition of Assets					$ (5,200)
Spare engines | Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased off lease | aircraft						1