Impact of COVID-19 - Balance Sheet, Cash Flow and Liquidity (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	May 06, 2020	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2020	May 18, 2020	Apr. 20, 2020	Mar. 30, 2020
Unusual or Infrequent Item, or Both [Line Items]
Unrestricted cash and short-term investments			$ 894,400,000
Maintenance, materials and repairs			$ 34,076,000	$ 31,604,000
2022 Revolving Credit Facility | Line of Credit | Revolving credit facilities
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity								$ 110,000,000.0
Subsequent Event | 2022 Revolving Credit Facility | Line of Credit | Revolving credit facilities
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity	$ 165,000,000.0						$ 135,000,000.0
Proceeds from revolving credit facility		$ 135,000,000.0
Subsequent Event | Chief Executive Officer And President
Unusual or Infrequent Item, or Both [Line Items]
Base salary reduction	30.00%
Subsequent Event | Forecast
Unusual or Infrequent Item, or Both [Line Items]
Reduction of planned capital spending					$ (50,000,000)
Maintenance, materials and repairs					$ (20,000,000)
Subsequent Event | Forecast | Aircraft
Unusual or Infrequent Item, or Both [Line Items]
Reduction of planned capital spending	$ (185,000,000)
Subsequent Event | Forecast | 2022 Revolving Credit Facility | Line of Credit | Revolving credit facilities
Unusual or Infrequent Item, or Both [Line Items]
Line of credit facility, maximum borrowing capacity						$ 165,000,000.0
Subsequent Event | Forecast | Minimum
Unusual or Infrequent Item, or Both [Line Items]
Reduction of planned non-fuel operating costs	(20,000,000)
Subsequent Event | Forecast | Maximum
Unusual or Infrequent Item, or Both [Line Items]
Reduction of planned non-fuel operating costs	$ (30,000,000)