Impact of COVID-19 - CARES Act Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended				6 Months Ended		9 Months Ended
	Apr. 20, 2020	Sep. 30, 2020	Dec. 31, 2020	Sep. 30, 2020	Jul. 31, 2020	Sep. 30, 2019	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Unusual or Infrequent Item, or Both [Line Items]
Special credits				$ (148,308,000)		$ 0			$ (300,219,000)	$ 0
Employee tax credits due to COVID-19				7,800,000				$ 35,800,000
Tax loss carrybacks due to COVID-19									140,800,000
Federal fuel tax savings due to COVID-19									3,400,000
Additional liquidity expected due to deferred payment of employer portion of social security taxes from CARES Act									23,000,000
Deferred payment of employer portion of social security taxes from CARES Act		$ 17,600,000		$ 17,600,000				$ 17,600,000	$ 17,600,000
Payroll Support Program
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program					$ 334,700,000
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)	500,151	20,646		20,646				20,646	20,646
Strike price (USD per share)	$ 14.08	$ 14.08		$ 14.08				$ 14.08	$ 14.08
Warrants, fair value	$ 3,700,000	$ 200,000		$ 200,000				$ 200,000	$ 200,000
Number of securities called by warrants (in shares)		500,151		500,151				500,151	500,151
Special credits				$ 142,900,000					$ 266,800,000
Payroll Support Program, Grant
Unusual or Infrequent Item, or Both [Line Items]
Proceeds received from Payroll Support Program		$ 6,600,000			260,600,000
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%		1.00%				1.00%	1.00%
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	$ 2,900,000		$ 2,900,000	$ 70,400,000			$ 2,900,000	$ 2,900,000
Term	10 years	10 years			10 years
Notes payable To United States Treasury | Payroll Support Program Loan
Unusual or Infrequent Item, or Both [Line Items]
Principal amount		$ 9,700,000		$ 9,700,000				$ 9,700,000	$ 9,700,000
Forecast
Unusual or Infrequent Item, or Both [Line Items]
Employee tax credits due to COVID-19			$ 2,000,000
Federal fuel tax savings due to COVID-19							$ 3,000,000