Impact of COVID-19 - Balance Sheet, Cash Flow and Liquidity Narrative (Details)		3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	May 12, 2020 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) aircraft	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) aircraft	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2021 USD ($)	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		$ 1,233,800,000		$ 1,233,800,000
Long-term debt		2,664,800,000		$ 2,664,800,000
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				9
Number of aircraft to be delivered, remainder of fiscal year | aircraft		3		3
Number of aircraft to be delivered, 2021 | aircraft		16		16
Maintenance, materials and repairs		$ 19,227,000	$ 34,688,000	$ 53,303,000	$ 66,292,000
A-320-Neo
Unusual or Infrequent Item, or Both [Line Items]
Number of aircraft with secured debt financing commitments scheduled for delivery | aircraft		20		20
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
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		$ 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
Debt issuance costs		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		111,200,000
Proceeds from revolving credit facility				111,200,000
Repayments of lines of credit				$ 48,800,000
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						$ 50,000,000
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