Debt (Term Facility and Revolving Credit Facility) (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Line of Credit Facility [Line Items]
Unamortized discount (premium) and debt issuance costs, net		$ 8,911		$ 9,436
Total debt		887,552		885,470
Amortization of debt issuance costs and discounts		526	$ 515
Line of credit facility, remaining borrowing capacity		45,000		45,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, remaining borrowing capacity		125,000		125,000
First Lien Term Facility, due December 20, 2024
Line of Credit Facility [Line Items]
Unamortized discount (premium) and debt issuance costs, net		(8,911)		(9,436)
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]	(2,130)		(2,112)
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	[2]	$ (6,781)		(7,324)
First Lien Term Facility, due December 20, 2024 | Term Loan
Line of Credit Facility [Line Items]
Debt instrument, interest rate, stated percentage		2.65%
Debt instrument, periodic payment, principal		$ 2,369
London Interbank Offered Rate (LIBOR) | First Lien Term Facility, due December 20, 2024 | Term Loan
Line of Credit Facility [Line Items]
Debt instrument, interest rate, stated percentage		2.50%
Debt instrument, basis spread on variable rate		0.15%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Total debt		$ 0		0
First Lien Term Facility, due December 20, 2024
Line of Credit Facility [Line Items]
Total debt		$ 816,907		$ 819,276

[1] Included in Current portion of debt, net of deferred financing fees on the Consolidated Balance Sheets.

[2] Included in Long-term debt, net of deferred financing fees on the Consolidated Balance Sheets.