Special Charges (Details) $ in Thousands		3 Months Ended					12 Months Ended
	Mar. 28, 2018 USD ($) aircraft	Mar. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) aircraft	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Special charges		$ 89,168	$ 0	$ 7,853	$ 0	$ 4,776	$ 12,629	$ 37,189
Ratification Incentive		80,700
Adjustments for contractual provisions		8,500
Special charges, non-operating		9,201				0
Purchase of property and equipment		$ 237,221				111,141
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
Airbus A319 [Member]
Property, Plant and Equipment [Line Items]
Purchase price	$ 285,000
Capital Lease Obligations [Member] | Airbus A319 [Member]
Property, Plant and Equipment [Line Items]
Number of aircrafts purchased | aircraft	14