Leases and Prepaid Maintenance Deposits - Operating Leases (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) aircraft_engine aircraft	Dec. 31, 2016 USD ($) aircraft	Dec. 31, 2015 USD ($)
Operating Leased Assets [Line Items]
Total rental expense | $	$ 309,800	$ 283,900	$ 282,700
Rental expense charged to operations for aircraft and engine operating leases | $	205,852	201,675	211,531
Maintenance reserves, supplemental rent | $	400	2,200	2,300
Aircraft maintenance deposits | $	$ 326,200	286,400
Number of Spare Engines Capitalized	4
Number of leased engines delivered | aircraft_engine	1
Lease term	3 years
Spare engines
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft_engine	11
A320 Family
Operating Leased Assets [Line Items]
Number of Aircraft Held	112
A320 Family | Aircraft
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases	58
Fixed Maintenance Reserve Payments
Operating Leased Assets [Line Items]
Committed expenditures, 2018 | $	$ 8,400
Committed expenditures, 2019 | $	5,900
Committed expenditures, 2020 | $	5,600
Committed expenditures, 2021 | $	5,700
Committed expenditures, 2022 | $	4,900
Committed expenditures, 2023 and beyond | $	12,900
Aircraft Supplemental Rent
Operating Leased Assets [Line Items]
Supplemental rent expense | $	$ 8,400	$ 9,000	$ 7,700
Aircraft
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased	1
Number of aircraft and spare engines financed under operating leases with a variable rate	1
Number of aircrafts delivered under secured debt arrangements	17
Aircraft | A320 Family
Operating Leased Assets [Line Items]
Number of aircraft capitalized	54
Number of previously leased aircraft purchased	8
Aircraft | Airbus A319
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased		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	2
Spare engines | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	2 years
Spare engines | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	13 years
Third Party Lessor
Operating Leased Assets [Line Items]
Number of direct lease aircraft from a third-party lessor	2
Third Party Lessor | Aircraft
Operating Leased Assets [Line Items]
Number of aircrafts returned	2
Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased	1
Spare engines | Spare engines
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased	1