Special Charges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) aircraft	Dec. 31, 2016 USD ($) aircraft	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Purchase of property and equipment	$ 628,881	$ 541,122	$ 548,800
Special charges	$ 12,629	$ 37,189	673
Restructuring Costs			$ 700
Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | aircraft	1
Purchase of property and equipment	$ 12,600
Value of noncash consideration	7,400
Fair value of assets acquired	11,900
Special charges	7,900
Aircraft | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration	200
Airbus A319 | Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | aircraft		7
Purchase price	$ 20,000	$ 147,700
Purchase of property and equipment		107,100
Value of noncash consideration		40,600
Fair value of assets acquired		95,700
Special charges		37,200
Airbus A319 | Aircraft | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration		$ 14,800
Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | aircraft	1
Spare engines | Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased | aircraft	1
Purchase price	$ 8,100
Purchase of property and equipment	3,800
Value of noncash consideration	4,300
Fair value of assets acquired	3,100
Special charges	4,800
Spare engines | Spare engines | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration	$ 200