Subsequent Events (Details) - USD ($)		6 Months Ended
	Apr. 01, 2021	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Subsequent Event [Line Items]
Borrowings under credit facility		$ 0	$ 2,597,000
Total debt		889,427,000		$ 885,470,000
Write off of Deferred Debt Issuance Cost		0	$ 1,795,000
First Lien Term Facility, due December 20, 2024
Subsequent Event [Line Items]
Total debt		$ 814,538,000		$ 819,276,000
Subsequent Event
Subsequent Event [Line Items]
Unamortized Debt Issuance Expense	$ 822,000
Accounts Receivable from Securitization	142,200,000
Subsequent Event | water consulting specialists, inc
Subsequent Event [Line Items]
Cash paid at closing	12,025,000
Subsequent Event | Maximum
Subsequent Event [Line Items]
Accounts Receivable from Securitization	150,000,000
Subsequent Event | Term Loan
Subsequent Event [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
Subsequent Event | Term Loan | 2021 Credit Agreement
Subsequent Event [Line Items]
Amount of loan	475,000,000
Subsequent Event | Term Loan | Interest Expense [Member]
Subsequent Event [Line Items]
Debt Issuance Costs, Gross	3,054,000
Subsequent Event | First Lien Term Facility, due December 20, 2024
Subsequent Event [Line Items]
Write off of Deferred Debt Issuance Cost	1,333,000
Revolving Credit Facility | Subsequent Event
Subsequent Event [Line Items]
Borrowings under credit facility	105,000,000
Revolving Credit Facility | Subsequent Event | 2021 Credit Agreement
Subsequent Event [Line Items]
Credit facility borrowing capacity	350,000,000
Letter of Credit | Subsequent Event | 2021 Credit Agreement
Subsequent Event [Line Items]
Credit facility borrowing capacity	$ 60,000,000