Commitments and Contingencies - Employees (Details) $ in Millions	1 Months Ended			12 Months Ended
	Jan. 31, 2018 USD ($)	May 31, 2016	Mar. 31, 2016	Dec. 31, 2017 USD ($) employee_group	Dec. 31, 2016 USD ($) employee_group	Dec. 31, 2015 USD ($)
Health Insurance Product Line
Concentration Risk [Line Items]
Accrued health care claims				$ 3.9	$ 5.7
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
Unionized employees concentration risk | Number of employees (percent)
Concentration Risk [Line Items]
Union-represented employee groups | employee_group				4	4
Concentration of risk				75.00%	73.00%
Unionized employees concentration risk | Number of employees (percent) | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Concentration of risk				26.00%
Unionized employees concentration risk | Number of employees (percent) | Association of Flight Attendants (AFA-CWA)
Concentration Risk [Line Items]
Concentration of risk				44.00%
Unionized employees concentration risk | Number of employees (percent) | Transport Workers Union (TWU)
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%
Subsequent Event
Concentration Risk [Line Items]
Ratification Incentive	$ 75.0
Subsequent Event | Air Line Pilots Association, International (ALPA)
Concentration Risk [Line Items]
Tentative collective bargaining agreement, contract term	5 years