Impact of COVID-19 - Balance Sheet, Cash Flow and Liquidity Narrative (Details)		2 Months Ended	3 Months Ended		5 Months Ended	12 Months Ended
	May 12, 2020 USD ($) $ / shares shares	Sep. 30, 2020 shares	Dec. 31, 2020 USD ($) aircraft_Engine aircraft	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) aircraft_Engine aircraft	Dec. 31, 2020 USD ($) aircraft aircraft_Engine shares	Mar. 31, 2021 USD ($)	Sep. 17, 2020 USD ($)	Jul. 22, 2020 shares	Jun. 30, 2020 USD ($)	Mar. 30, 2020 USD ($)	Mar. 29, 2020 $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Cash, cash equivalents and short-term investments			$ 1,896,100,000		$ 1,896,100,000	$ 1,896,100,000
Long-term debt			$ 3,580,500,000		$ 3,580,500,000	3,580,500,000
Equity component value of convertible debt issuance, net						$ 55,590,000
Purchase price (in dollars per share) | $ / shares												$ 60.00
Number of aircraft delivered | aircraft						12
Number of aircraft with pre-delivery deposit payments scheduled for delivery | aircraft			16		16	16
Debt discount			$ 131,400,000		$ 131,400,000	$ 131,400,000							$ 40,400,000
A320NEO
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery, pledged as collateral | aircraft_Engine			11		11	11
Over-Allotment Option | Common Stock
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) | shares	2,625,000
At-The-Market Offering Program
Unusual or Infrequent Item, or Both [Line Items]
Number of shares sold (in shares) | shares		9,000,000				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
Public Offering | Common Stock
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 (in dollars per share) | $ / shares	$ 10.00
Payments of stock issuance costs	$ 8,900,000
2020 Revolving Credit Facility | Line of Credit | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity										$ 180,000,000.0	$ 110,000,000.0
Proceeds from lines of credit						$ 180,000,000.0
4.75% Convertible Senior Notes | Convertible Debt
Unusual or Infrequent Item, or Both [Line Items]
Debt face amount	$ 175,000,000.0
Stated interest rate percentage	4.75%
Proceeds from 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, net			3,800,000		3,800,000	3,800,000
Equity component value of convertible debt issuance, net						72,700,000
Payment of 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]
Maximum borrowing capacity														$ 160,000,000.0
Proceeds from lines of credit				$ 160,000,000.0
2020 Amended Revolving Credit Facility, Due 2021 | Line of Credit | Revolving Credit Facility
Unusual or Infrequent Item, or Both [Line Items]
Maximum borrowing capacity			95,100,000		95,100,000	95,100,000				$ 111,200,000
Proceeds from lines of credit						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]
Debt face amount								$ 850,000,000
Stated interest rate percentage								8.00%
Debt issuance costs, net			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]
Maximum borrowing capacity							$ 54,000,000