PENSION AND OTHER POSTRETIREMENT PLANS - Net Periodic Pension and Other Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,919	$ 8,679	$ 7,998
Interest cost	6,145	7,735	9,202
Expected return on plan assets	(10,809)	(10,214)	(10,304)
Prior service cost	(127)	(186)	(186)
Net actuarial loss	9,769	9,767	4,245
Curtailment gain	(220)	0	0
Special termination benefits	315	0	0
Prior-service cost write-off	261	0	0
Net benefit cost	13,253	15,781	10,955
Other Postretirement
Defined Benefit Plan Disclosure [Line Items]
Service cost	201	227	244
Interest cost	376	501	718
Expected return on plan assets	0	0	0
Prior service cost	(364)	(464)	(195)
Net actuarial loss	0	0	(59)
Curtailment gain	0	0	0
Special termination benefits	0	0	0
Prior-service cost write-off	0	0	0
Net benefit cost	$ 213	$ 264	$ 708