Special Charges (Details) $ in Thousands	3 Months Ended						12 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($) aircraft	Dec. 31, 2016 USD ($) aircraft
Property, Plant and Equipment [Line Items]
Purchase of property and equipment							$ 606,971	$ 628,881	$ 539,209
Special charges			$ 0	$ 7,853	$ 0	$ 4,776	88,921	12,629	37,189
Special charges, non-operating							$ 90,357	$ 0	$ 0
Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft							14	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
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment Purchased, Consideration Transferred	$ 285,000	$ 285,000
Airbus A319 | Aircraft
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | 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 off lease | aircraft							1
Spare engines | Spare engines
Property, Plant and Equipment [Line Items]
Number of previously leased aircraft purchased off lease | aircraft								1
Purchase of property and equipment								$ 3,800
Value of noncash consideration								4,300
Fair value of assets acquired								3,100
Special charges								4,800
Property, Plant and Equipment Purchased, Consideration Transferred								8,100
Spare engines | Spare engines | Other Non-Cash Items
Property, Plant and Equipment [Line Items]
Value of noncash consideration								$ 200
Capital Lease Obligations | Airbus A319
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Number Of Aircraft Purchased | aircraft		14
Pilots
Property, Plant and Equipment [Line Items]
Ratification Incentive							$ 80,200
Collective Bargaining Agreement, Contractual Provision Adjustments							8,500
Special charges							$ 88,700