Long-Term Debt (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Oct. 21, 2019
Debt Instrument
Long-term and short-term debt, combined amount	$ 627,400,000		$ 627,400,000		$ 703,700,000
Long-term Debt, Current Maturities	(73,200,000)		(73,200,000)		(63,800,000)
Unamortized Debt Issuance Expense	(5,600,000)		(5,600,000)		(6,600,000)
Debt Instrument, Unamortized Discount	(9,000,000.0)		(9,000,000.0)		(10,700,000)
Long-term debt	539,600,000		539,600,000		622,600,000
Loss on refinancing of debt	1,500,000	$ 0	1,500,000	$ 0
Term Loan
Debt Instrument
Long-term debt, gross	627,400,000		627,400,000		673,700,000	$ 700,000,000.0
Periodic payment of long-term debt principal			8,750,000
Excess cash flow prepayment			28,800,000
Excess cash flow prepayment, current maturies of debt	38,200,000		38,200,000
Loss on refinancing of debt	1,500,000		1,500,000
Letter of Credit
Debt Instrument
Maximum borrowing capacity on line of credit	20,000,000.0		20,000,000.0
Revolving Credit Facility
Debt Instrument
Long-term debt, gross	0		0		30,000,000.0
Maximum borrowing capacity on line of credit	200,000,000.0		200,000,000.0			$ 200,000,000.0
Proceeds from borrowing under line of credit			20,000,000.0	185,000,000.0
Repayments of lines of credit			50,000,000.0	$ 65,000,000.0
Remaining borrowing capacity on line of credit	200,000,000.0		200,000,000.0		170,000,000.0
Letters of credit outstanding, amount	0		0		$ 0
Debt covenant, maximum undischarged judgments	$ 65,000,000.0		$ 65,000,000.0