Impact of the COVID-19 Pandemic (Details) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended
	May 10, 2021 USD ($)	Jun. 30, 2021 USD ($) item $ / shares shares	Jun. 30, 2020 item	Jun. 30, 2019 item	Jun. 30, 2021 USD ($) $ / shares shares
Unusual or Infrequent Item, or Both [Line Items]
Number of flights operated | item		185,498	80,755	215,052
Flights increase percentage		129.70%
Liquid assets		$ 996.7			$ 996.7
Cash and marketable securities		955.7			955.7
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	$ 61.2
PSP Extension Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		268.1			$ 268.1
PSP 3 Agreement
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		250.0			250.0
PSP 3 Agreement | Other noncurrent liabilities
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		115.3			115.3
PSP 3 Agreement | Operating expenses
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		89.7			89.7
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]
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]
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