Long-term Debt - Schedule of Long-term Debt (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Obligations under finance leases	$ 99,800,000		$ 99,800,000		$ 86,500,000
Total long-term debt	431,800,000		431,800,000		344,500,000
Unamortized debt issuance costs	3,300,000		3,300,000		900,000
Amortization of debt issuance costs			500,000	$ 400,000
Revolving credit facility
Debt Instrument [Line Items]
Amounts borrowed (Credit Facility)	332,000,000.0		332,000,000.0		258,000,000.0
Total long-term debt	332,000,000.0		332,000,000.0		258,000,000.0
Outstanding letters of credit	43,800,000		43,800,000		19,500,000
Amounts available to borrow	347,300,000		347,300,000		$ 402,400,000
Revolving credit facility, expansion feature	200,000,000.0		200,000,000.0
Average borrowings	221,200,000	$ 354,300,000	211,900,000	345,300,000
Unused Credit Facility and letter of credit participation fees	400,000	300,000	800,000	600,000
Amortization of debt issuance costs	$ 200,000	$ 200,000	$ 500,000	$ 400,000
Weighted-average interest rate	1.40%	1.40%	1.40%	2.10%
Minimum | Revolving credit facility
Debt Instrument [Line Items]
Average borrowings	$ 126,000,000.0	$ 225,000,000.0	$ 126,000,000.0	$ 151,500,000
Maximum | Revolving credit facility
Debt Instrument [Line Items]
Average borrowings	$ 378,000,000.0	$ 451,000,000.0	$ 378,000,000.0	$ 499,300,000