Leases and Prepaid Maintenance Deposits - Operating Leases (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) aircraft_engine aircraft	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Operating Leased Assets [Line Items]
Total rental expense	$ 282,700	$ 254,300	$ 214,600
Rental expense charged to operations for aircraft and engine operating leases	211,531	195,827	169,737
Maintenance reserves, supplemental rent	2,300	1,600	1,900
Aircraft maintenance deposits	279,900	250,000
Refund in pre-delivery deposits	$ 48,400
Spare engines
Operating Leased Assets [Line Items]
Number of aircraft and spare engines financed under operating leases | aircraft_engine	11
A320 Family [Member]
Operating Leased Assets [Line Items]
Number of Aircraft Held | aircraft	79
Fixed Maintenance Reserve Payments
Operating Leased Assets [Line Items]
Committed expenditures, 2016	$ 8,000
Committed expenditures, 2017	7,400
Committed expenditures, 2018	5,800
Committed expenditures, 2019	4,200
Committed expenditures, 2020	3,900
Committed expenditures, 2021 and beyond	10,200
Aircraft rent
Operating Leased Assets [Line Items]
Supplemental rent expense	$ 7,700	$ 7,500	$ 5,200
Aircraft
Operating Leased Assets [Line Items]
Number of aircraft and spare engines financed under operating leases with a variable rate | aircraft	5
Aircraft | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	3 years
Aircraft | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	15 years
Other leased equipment and property | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	25 years
Spare engines
Operating Leased Assets [Line Items]
Number of spare aircraft engines entered into sale and leaseback agreement | aircraft_engine	1
Spare engines | Minimum
Operating Leased Assets [Line Items]
Operating leases, term	7 years
Spare engines | Maximum
Operating Leased Assets [Line Items]
Operating leases, term	12 years