LONG-TERM DEBT - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023	Sep. 30, 2020
Debt Instrument [Line Items]
Long-term debt, Outstanding Balance	$ 1,458,736	$ 1,489,812
less: Current portion, Outstanding Balance	(9,274)	(9,625)
Long-term debt, Outstanding Balance, Non-current	1,449,462	1,480,187
Original Issuer Premium/(Discount)	(674)	(704)
Capitalized Fees & Expenses	(18,553)	(19,579)
Long-term debt, Balance Sheet	1,439,509	1,469,529
less: Current portion, Balance Sheet	(9,274)	(9,625)
Long-term debt, Balance Sheet, Non-current	1,430,235	1,459,904
Senior notes due 2028
Debt Instrument [Line Items]
Long-term debt, Outstanding Balance	974,775	974,775
Original Issuer Premium/(Discount)	206	218
Capitalized Fees & Expenses	(8,415)	(8,920)
Long-term debt, Balance Sheet	$ 966,566	$ 966,073
Coupon Interest Rate	5.75%	5.75%	5.75%
Term Loan B due 2029
Debt Instrument [Line Items]
Long-term debt, Outstanding Balance	$ 461,000	$ 463,000
Original Issuer Premium/(Discount)	(880)	(922)
Capitalized Fees & Expenses	(6,708)	(7,039)
Long-term debt, Balance Sheet	453,412	455,039
Revolver due 2025
Debt Instrument [Line Items]
Long-term debt, Outstanding Balance	21,500	50,445
Original Issuer Premium/(Discount)	0	0
Capitalized Fees & Expenses	(3,419)	(3,606)
Long-term debt, Balance Sheet	18,081	46,839
Non US lines of credit
Debt Instrument [Line Items]
Long-term debt, Outstanding Balance	0	0
Original Issuer Premium/(Discount)	0	0
Capitalized Fees & Expenses	0	(3)
Long-term debt, Balance Sheet	0	(3)
Other long term debt
Debt Instrument [Line Items]
Long-term debt, Outstanding Balance	1,461	1,592
Original Issuer Premium/(Discount)	0	0
Capitalized Fees & Expenses	(11)	(11)
Long-term debt, Balance Sheet	$ 1,450	$ 1,581