Special Charges (Details) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) aircraft	Sep. 30, 2017 USD ($) aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Ratification Incentive							$ 80,200
Adjustments for contractual provisions							8,500
Special charges (credits)	$ (686)		$ 0	$ 7,853	$ 0	$ 4,776	88,656	$ 12,629	$ 12,629	$ 37,189
Ratification Incentive Paid							76,200
Purchase of property and equipment							420,586	428,913
Special charges, non-operating	$ 1,744			$ 0			$ 90,357	0
Airbus A319 [Member]
Property, Plant and Equipment [Line Items]
Purchase price		$ 285,000
Airbus A319 [Member] | Capital Lease Obligations [Member]
Property, Plant and Equipment [Line Items]
Number of aircrafts purchased | aircraft		14
Aircraft [Member]
Property, Plant and Equipment [Line Items]
Special charges (credits)								$ 7,900
Number of previously leased engines | aircraft							22	1
Purchase of property and equipment								$ 12,600
Value of noncash consideration								7,400
Fair value of assets acquired								11,900
Aircraft [Member] | Other Non-Cash Items [Member]
Property, Plant and Equipment [Line Items]
Value of noncash consideration								200
Aircraft [Member] | Airbus A319 [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment To Be Purchased, Consideration Transferred								20,000
Spare Engines [Member]
Property, Plant and Equipment [Line Items]
Number of previously leased engines | aircraft							1
Spare Engines [Member] | Spare Engines [Member]
Property, Plant and Equipment [Line Items]
Special charges (credits)								$ 4,800
Number of previously leased engines | 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
Spare Engines [Member] | Spare Engines [Member] | Other Non-Cash Items [Member]
Property, Plant and Equipment [Line Items]
Value of noncash consideration								$ 200