Long-term Debt - Schedule of Long-term Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Obligations under finance leases	$ 86,500,000	$ 57,300,000
Total long-term debt	344,500,000	382,100,000
Outstanding letters of credit	19,500,000
Unused credit facility and letter of credit participation	1,400,000	1,200,000	$ 1,200,000
Charges related to amortization of debt issuance costs	800,000	800,000	$ 800,000
Revolving Credit Facility | Minimum
Debt Instrument [Line Items]
Average borrowings	69,000,000.0	141,700,000
Revolving Credit Facility | Maximum
Debt Instrument [Line Items]
Average borrowings	499,300,000	508,000,000.0
Revolving Credit Facility
Debt Instrument [Line Items]
Amounts borrowed (Credit Facility)	258,000,000.0	324,800,000
Outstanding letters of credit	19,500,000	16,700,000
Amounts available to borrow	402,400,000	341,700,000
Average borrowings	259,500,000	303,200,000
Unamortized debt issuance costs	$ 900,000	$ 1,700,000
Weighted-average interest rate (percent)	1.80%	3.40%
Revolving credit facility, expansion feature	$ 200,000,000.0