Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended				12 Months Ended
	Oct. 31, 2018	Feb. 28, 2018	May 31, 2016	Mar. 31, 2016	Dec. 31, 2018 USD ($) employee_group	Dec. 31, 2017 USD ($) employee_group	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Health Insurance Product Line
Concentration Risk [Line Items]
Accrued health care claims					$ 4.4	$ 3.9
Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Collective Bargaining Arrangement, Term		5 years
Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Tentative collective bargaining agreement, contract term				5 years
Ratified collective bargaining agreement, contract term			5 years
Ratified Collective Bargaining Agreement, Incentive Payment							$ 9.6
Increase (Decrease) in Employee Related Liabilities to Ratified Collective Bargaining Agreement, Incentive Payment								$ 1.2
Association of Flight Attendants (AFA-CWA) | Salaries, Wages, and Benefits
Concentration Risk [Line Items]
Increase (Decrease) in Employee Related Liabilities to Ratified Collective Bargaining Agreement, Incentive Payment							$ 8.4
Professional Airline Flight Control Association (PAFCA)
Concentration Risk [Line Items]
Collective Bargaining Arrangement, Term	5 years
Unionized employees concentration risk | Number of employees (percent)
Concentration Risk [Line Items]
Union-represented employee groups | employee_group					5	4
Concentration of risk					80.00%	75.00%
Unionized employees concentration risk | Number of employees (percent) | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Concentration of risk					27.00%
Unionized employees concentration risk | Number of employees (percent) | Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Concentration of risk					45.00%
Unionized employees concentration risk | Number of employees (percent) | Professional Airline Flight Control Association (PAFCA)
Concentration Risk [Line Items]
Concentration of risk					1.00%
Unionized employees concentration risk | Number of employees (percent) | International Association of Machinists and Aerospace Workers (IAMAW)
Concentration Risk [Line Items]
Concentration of risk					4.00%
Unionized employees concentration risk | Number of employees (percent) | Transport Workers Union of America (TWU)
Concentration Risk [Line Items]
Concentration of risk					3.00%