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Impact of COVID-19 - Balance Sheet, Cash Flow and Liquidity Narrative (Details)		2 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	May 12, 2020 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) aircraft shares	Sep. 30, 2020 USD ($) aircraft	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) aircraft	Dec. 31, 2019 USD ($)	Jun. 30, 2021 USD ($)	Sep. 17, 2020 USD ($) shares	Jul. 22, 2020 shares	Mar. 31, 2020 USD ($)	Mar. 30, 2020 USD ($)	Mar. 29, 2020 $ / shares	Dec. 31, 2018 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Cash, cash equivalents and short-term investments		$ 2,053,900,000	$ 2,053,900,000			$ 2,053,900,000
Long-term debt		$ 3,584,800,000	$ 3,584,800,000			$ 3,584,800,000
Equity component value of convertible debt issuance, net				$ 55,590,000
Purchase price (USD per share) \| $ / shares													$ 60.00
Payments of stock issuance costs	$ 8,900,000
Number of aircraft delivered \| aircraft						10
Number of aircraft to be delivered, remainder of fiscal year \| aircraft		2	2			2
Number of aircraft with pre-delivery deposit payments scheduled for delivery \| aircraft		16	16			16
Debt discount		$ 60,000,000.0	$ 60,000,000.0			$ 60,000,000.0	$ 40,400,000
A-320-Neo
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery \| aircraft		19	19			19
Public Stock Offering
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) \| shares	20,125,000
Consideration received on sale of shares	$ 192,400,000
Purchase price (USD per share) \| $ / shares	$ 10.00
Over-Allotment Option
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) \| shares	2,625,000
ATM Program
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) \| shares		9,000,000
Consideration received on sale of shares		$ 156,700,000
Payments of stock issuance costs		$ 5,000,000.0
Shares available for sale (in shares) \| shares									9,000,000	9,000,000
8.00% senior secured notes due in 2025
Unusual or Infrequent Item, or Both [Line Items]
Stated interest rate		8.00%	8.00%			8.00%
2022 Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity				$ 180,000,000.0							$ 110,000,000.0	$ 110,000,000.0
Proceeds from revolving credit facility						$ 180,000,000.0
4.75% Convertible Senior Notes \| Convertible debt
Unusual or Infrequent Item, or Both [Line Items]
Principal amount	$ 175,000,000.0
Stated interest rate	4.75%
Proceeds from issuance of convertible debt	$ 168,300,000
Payments of financing costs	$ 6,700,000
Long-term debt		$ 95,600,000	$ 95,600,000			95,600,000
Debt issuance costs		$ 3,800,000	3,800,000			3,800,000
Equity component value of convertible debt issuance, net			$ 72,700,000
Common stock and warrants issuance costs						2,900,000
2020 Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity														$ 160,000,000.0
Proceeds from revolving credit facility							$ 160,000,000.0
Revolving credit facility due in 2021 \| Line of Credit \| Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity											$ 111,200,000
Proceeds from revolving credit facility						$ 95,100,000
8.00% Senior Secured Notes Due 2025 \| 8.00% senior secured notes due in 2025
Unusual or Infrequent Item, or Both [Line Items]
Principal amount									$ 850,000,000
Stated interest rate		8.00%	8.00%			8.00%			8.00%
Debt issuance costs		$ 17,400,000	$ 17,400,000			$ 17,400,000
Proceeds from Issuance of Secured Debt			823,900,000
Debt discount		$ 8,700,000	$ 8,700,000			$ 8,700,000
Subsequent Event \| 2020 Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity								$ 54,000,000
Subsequent Event \| Scenario, Plan
Unusual or Infrequent Item, or Both [Line Items]
Reduction in planned capital spending due to COVID-19					$ 65,000,000
Reduction in planned heavy maintenance events due to COVID-19					20,000,000
Subsequent Event \| Scenario, Plan \| Minimum
Unusual or Infrequent Item, or Both [Line Items]
Reduction in planned non-fuel operating costs due to COVID-19					20,000,000
Subsequent Event \| Scenario, Plan \| Maximum
Unusual or Infrequent Item, or Both [Line Items]
Reduction in planned non-fuel operating costs due to COVID-19					$ 30,000,000