Impact of the COVID-19 Pandemic (Details) $ / shares in Units, $ in Millions		3 Months Ended				9 Months Ended
	May 10, 2021 USD ($)	Sep. 30, 2021 USD ($) item $ / shares shares	Jun. 30, 2021 USD ($)	Sep. 30, 2020 item	Sep. 30, 2019 item	Sep. 30, 2021 USD ($) $ / shares shares
Unusual or Infrequent Item, or Both [Line Items]
Number of flights operated | item		210,251		137,493	219,272
Flights increase percentage		52.90%
Liquid assets		$ 953.5				$ 953.5
Cash and marketable securities		912.5				912.5
Line of credit facility, reduction of amount available		$ 41.0				$ 41.0
Warrants to purchase shares | shares		78,000				78,000
Warrants exercise price | $ / shares		$ 57.47				$ 57.47
Repayments of secured debt	$ 61.2	$ 80.1	$ 61.2
PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		268.1				$ 268.1
PSP Extension Agreement | Operating expenses
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		217.7				217.7
PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		250.0				250.0
PSP 3 Agreement | Operating expenses
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		205.0				205.0
Grant | PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		217.7				217.7
Grant | PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		205.0				205.0
Unsecured term loan | PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		$ 50.4				$ 50.4
Debt term		10 years				10 years
Unsecured term loan | PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		$ 45.0				$ 45.0
Debt term		10 years				10 years
Unsecured term loan | First five years | PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Debt term		5 years				5 years
Interest rate (as a percent)						1.00%
Unsecured term loan | First five years | PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Debt term		5 years				5 years
Interest rate (as a percent)						1.00%
Unsecured term loan | Final five years | PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Debt term		5 years				5 years
Interest rate (as a percent)						2.00%
Unsecured term loan | Final five years | PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Debt term		5 years				5 years
Interest rate (as a percent)						2.00%
Warrant Shares | PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Warrants to purchase shares | shares		124,773				124,773
Warrants exercise price | $ / shares		$ 40.41				$ 40.41
Warrant term						5 years
Warrant Shares | PSP Extension Agreement | Maximum
Unusual or Infrequent Item, or Both [Line Items]
Warrants to purchase shares | shares		124,773				124,773
Warrant Shares | PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Warrants to purchase shares | shares		78,317				78,317
Warrants exercise price | $ / shares		$ 57.47				$ 57.47
Warrant term						5 years
Warrant Shares | PSP 3 Agreement | Maximum
Unusual or Infrequent Item, or Both [Line Items]
Warrants to purchase shares | shares		78,317				78,317