Leases and Prepaid Maintenance Deposits - Operating Leases (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) aircraft aircraft_engine	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Operating Leased Assets [Line Items]
Total rental expense	$ 283,900	$ 282,700	$ 254,300
Rental expense charged to operations for aircraft and engine operating leases	201,675	211,531	195,827
Maintenance reserves, supplemental rent	2,200	2,300	1,600
Aircraft maintenance deposits	$ 286,400	279,900
Number of aircraft capitalized | aircraft	36
Number of Spare Engines Capitalized | aircraft	1
Aircraft
Operating Leased Assets [Line Items]
Number of leased assets financed under operating leases | aircraft	59
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 | aircraft	95
Fixed Maintenance Reserve Payments
Operating Leased Assets [Line Items]
Committed expenditures, 2017	$ 7,700
Committed expenditures, 2018	7,000
Committed expenditures, 2019	5,700
Committed expenditures, 2020	5,400
Committed expenditures, 2021	5,500
Committed expenditures, 2022 and beyond	17,700
Aircraft Supplemental Rent
Operating Leased Assets [Line Items]
Supplemental rent expense	$ 9,000	$ 7,700	$ 7,500
Aircraft
Operating Leased Assets [Line Items]
Number of aircraft and spare engines financed under operating leases with a variable rate | aircraft	2
Number of aircrafts delivered under secured debt arrangements | aircraft	11
Aircraft | Airbus A319
Operating Leased Assets [Line Items]
Number of previously leased aircraft purchased | aircraft	7
Aircraft | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	12 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	1
Spare engines | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	10 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 | aircraft	5