Debt - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended
	Apr. 01, 2021	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Sep. 30, 2020
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity		$ 60,000,000		$ 60,000,000			$ 45,000,000
Borrowings under credit facility				0	$ 2,597,000
Unamortized discount (premium) and debt issuance costs, net	$ 2,375,000	12,217,000		12,217,000			9,436,000
Amortization of debt issuance costs and discounts		437,000	$ 277,000	1,481,000	1,213,000
Outstanding debt		801,861,000		801,861,000			885,470,000
Accounts receivable from securitization	142,200,000
Write off of deferred financing costs				1,333,000	$ 1,795,000
Maximum
Line of Credit Facility [Line Items]
Accounts receivable from securitization	150,000,000
Term Loan
Line of Credit Facility [Line Items]
Borrowings under term loan	475,000,000
Debt instrument, periodic payment, principal	1,188,000
Unamortized debt issuance expense	1,931,000
Payments of financing costs	4,985,000
Term Loan | Interest Expense
Line of Credit Facility [Line Items]
Debt issuance costs	3,054,000
2021 Term Loan
Line of Credit Facility [Line Items]
Outstanding debt		$ 475,000,000		$ 475,000,000		$ 814,538,000	0
Debt , interest rate, effective percentage		2.63%		2.63%
Debt instrument, basis spread on variable rate				2.50%
2021 Term Loan | LIBOR
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate				0.13%
First Lien Term Facility, due December 20, 2024
Line of Credit Facility [Line Items]
Outstanding debt		$ 0		$ 0			819,276,000
Write off of deferred financing costs	1,333,000
2021 Revolving Credit Facility, due April 2026
Line of Credit Facility [Line Items]
Outstanding debt		$ 95,000,000		$ 95,000,000			0
Debt , interest rate, effective percentage		2.38%		2.38%
Debt instrument, basis spread on variable rate				2.25%
2021 Revolving Credit Facility, due April 2026 | LIBOR
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate				0.13%
Securitization Facility, due April 1, 2024
Line of Credit Facility [Line Items]
Outstanding debt		$ 148,151,000		$ 148,151,000			0
Interest payable		51,000		51,000
2021 Credit Agreement | Term Loan
Line of Credit Facility [Line Items]
Amount of loan	475,000,000
First Lien Term Facility, due December 20, 2024
Line of Credit Facility [Line Items]
Unamortized discount (premium) and debt issuance costs, net		(12,217,000)		(12,217,000)			(9,436,000)
First Lien Term Facility, due December 20, 2024 | Other Current Liabilities
Line of Credit Facility [Line Items]
Unamortized discount (premium) and debt issuance costs, net		(1,791,000)		(1,791,000)			(2,112,000)
First Lien Term Facility, due December 20, 2024 | Other Noncurrent Liabilities
Line of Credit Facility [Line Items]
Unamortized discount (premium) and debt issuance costs, net		$ (10,426,000)		$ (10,426,000)			(7,324,000)
Securitization Facility, due April 1, 2024
Line of Credit Facility [Line Items]
Unamortized debt issuance expense	822,000
Debt , interest rate, effective percentage		1.35%		1.35%
Debt instrument, basis spread on variable rate				1.25%
Securitization Facility, due April 1, 2024 | LIBOR
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate				0.10%
Equipment Financing Due July 31 2029 | Loans Payable
Line of Credit Facility [Line Items]
Amount of loan		$ 8,378,000		$ 8,378,000
Unamortized debt issuance expense		$ 467,000		$ 467,000
Debt , interest rate, effective percentage		0.0475%		0.0475%
Debt instrument, basis spread on variable rate				3.75%
Equipment Financing Due July 31 2029 | Loans Payable | LIBOR
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate				1.00%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity		$ 350,000,000		$ 350,000,000			$ 125,000,000
Borrowings under credit facility	105,000,000
Revolving Credit Facility | 2021 Credit Agreement
Line of Credit Facility [Line Items]
Credit facility borrowing capacity	350,000,000
Letter of Credit | 2021 Credit Agreement
Line of Credit Facility [Line Items]
Credit facility borrowing capacity	$ 60,000,000