Impact of COVID-19 - COVID-19 Legislation Narrative (Details) - USD ($)			1 Months Ended		3 Months Ended	5 Months Ended	12 Months Ended
	Feb. 10, 2021	Dec. 31, 2020	Jan. 31, 2021	Dec. 31, 2020	Jul. 31, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 20, 2020
Unusual or Infrequent Item, or Both [Line Items]
Special charges (credits)							$ (302,761,000)	$ 717,000	$ 88,921,000
Tax loss carrybacks due to COVID-19							142,000,000.0
Federal fuel tax savings due to COVID-19							6,500,000
Deferred payment of employer portion of social security taxes from CARES Act		$ 23,200,000		$ 23,200,000			23,200,000
Unsecured term loans due in 2030
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program					$ 334,700,000	$ 344,400,000
Special charges (credits)							$ (266,800,000)
CARES Act
Unusual or Infrequent Item, or Both [Line Items]
Shares of common stock subject to warrants issued to the United States Treasury (in shares)										520,797
Strike price (in dollars per share)		$ 14.08		$ 14.08			$ 14.08			$ 14.08
Warrants, fair value										$ 3,900,000
Number of warrants issued as a percent of outstanding stock		1.00%		1.00%			1.00%
Payroll Support Program, Grant
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program						267,200,000
Payroll Support Program, Grant | Subsequent Event
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program			$ 92,200,000
Payroll Support Program 2 (PSP2)
Unusual or Infrequent Item, or Both [Line Items]
Proceeds expected from Payroll Support Program				$ 184,500,000
CARES Act Employee Retention
Unusual or Infrequent Item, or Both [Line Items]
Special charges (credits)							$ (38,500,000)
Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Number of warrants issued as a percent of outstanding stock		1.00%		1.00%			1.00%
Payroll Support Program | Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Debt face amount					$ 70,400,000	$ 2,900,000				$ 73,300,000
Debt term						10 years
Proceeds expected from Payroll Support Program		$ 25,000,000
Forecast | Payroll Support Program 2 (PSP2)
Unusual or Infrequent Item, or Both [Line Items]
Shares of common stock subject to warrants issued to the United States Treasury (in shares)	103,761
Strike price (in dollars per share)	$ 24.42
Forecast | Payroll Support Program | Payroll Support Program, Low-Interest Loan
Unusual or Infrequent Item, or Both [Line Items]
Debt term	10 years