Special Charges (Details) $ in Thousands		3 Months Ended						6 Months Ended		12 Months Ended
	Mar. 28, 2018 USD ($) aircraft	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) aircraft	Jun. 30, 2017 USD ($) aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Ratification Incentive			$ 80,700
Adjustments for contractual provisions			$ 8,500
Special charges		$ 174		$ 0	$ 7,853	$ 0	$ 4,776	$ 89,342	$ 4,776	$ 12,629	$ 37,189
Ratification Incentive Paid		75,800
Purchase of property and equipment								323,229	269,519
Special charges, non-operating		$ 79,412				$ 0		$ 88,613	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
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									$ 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