Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended	6 Months Ended
	Feb. 28, 2018	Jun. 30, 2018 USD ($) employee_group	Dec. 31, 2017 USD ($)
Air Line Pilots Association, International [Member]
Concentration Risk [Line Items]
Ratified collective bargaining agreement, contract term	5 years
Health Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Accrued health care claims | $		$ 5.1	$ 3.9
Unionized Employees Concentration Risk [Member] | Number of Employees, Total [Member]
Concentration Risk [Line Items]
Number of union-represented employee groups | employee_group		4
Company's employees covered under collective bargaining agreements (as a percent)		75.00%
Unionized Employees Concentration Risk [Member] | Number of Employees, Total [Member] | Air Line Pilots Association, International [Member]
Concentration Risk [Line Items]
Company's employees covered under collective bargaining agreements (as a percent)		27.00%
Unionized Employees Concentration Risk [Member] | Number of Employees, Total [Member] | Association of Flight Attendants [Member]
Concentration Risk [Line Items]
Company's employees covered under collective bargaining agreements (as a percent)		44.00%
Unionized Employees Concentration Risk [Member] | Number of Employees, Total [Member] | Professional Airline Flight Control Association [Member]
Concentration Risk [Line Items]
Company's employees covered under collective bargaining agreements (as a percent)		1.00%
Unionized Employees Concentration Risk [Member] | Number of Employees, Total [Member] | International Association Of Machinists And Aerospace Workers (IAM) [Member]
Concentration Risk [Line Items]
Company's employees covered under collective bargaining agreements (as a percent)		3.00%