Impact of COVID-19 - CARES Act Narrative (Details)				2 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2020 USD ($) shares	Apr. 29, 2020 USD ($)	Apr. 20, 2020 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) installment shares	Jun. 30, 2020 USD ($) installment shares	Jun. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($) installment shares	Jun. 30, 2019 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Maximum expected proceeds to be received from Payroll Support Program			$ 334,700,000
Shares of common stock subject to warrants issued to the United States Treasury (in shares) | shares			500,150	428,829	428,829			428,829
Strike price (USD per share) | $ / shares			$ 14.08
Proceeds from Payroll Support Program				$ 301,300,000
Special credits					$ (151,911,000)	$ 0		$ (151,911,000)	$ 0
Expected maximum availability from CARES Act Loan Program		$ 741,000,000
Employee tax credits due to COVID-19								28,000,000.0
Tax loss carrybacks due to COVID-19								140,800,000
Federal fuel tax savings due to COVID-19								800,000
Additional liquidity expected due to deferred payment of employer portion of social security taxes from CARES Act								24,000,000
Deferred payment of employer portion of social security taxes from CARES Act				$ 10,000,000.0	$ 10,000,000.0			$ 10,000,000.0
Payroll Support Program
Unusual or Infrequent Item, or Both [Line Items]
Number of installments received for Payroll Support Program | installment				3	3			3
Total number of Payroll Support Program installments | installment				4	4			4
CARES Act
Unusual or Infrequent Item, or Both [Line Items]
Adjustments to additional paid in capital, warrant Issued					$ 2,500,000
Amount recorded to deferred payroll liability for unused PSP proceeds due to COVID-19					237,900,000			$ 238,400,000
Special credits					123,900,000			123,900,000
Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Number of warrants issued as a percent of outstanding common stock		1.00%
Notes Payable To United States Treasury
Unusual or Infrequent Item, or Both [Line Items]
Principal amount				$ 60,400,000	60,400,000			60,400,000
Term				10 years
Notes Payable To United States Treasury | Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Principal amount			$ 70,400,000	$ 60,400,000	$ 60,400,000			$ 60,400,000
Term			10 years
Forecast
Unusual or Infrequent Item, or Both [Line Items]
Shares of common stock subject to warrants issued to the United States Treasury (in shares) | shares	71,321
Proceeds from Payroll Support Program	$ 33,400,000
Employee tax credits due to COVID-19							$ 10,000,000
Federal fuel tax savings due to COVID-19							$ 5,000,000
Forecast | Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Term	10 years
Forecast | Notes Payable To United States Treasury
Unusual or Infrequent Item, or Both [Line Items]
Principal amount	$ 10,000,000.0