EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2,056	$ 1,845	$ 6,167	$ 5,536
Interest cost	2,647	3,043	7,940	9,129
Expected return on plan assets	(5,145)	(4,763)	(15,434)	(14,290)
Recognized actuarial loss	2,606	1,442	7,818	4,324
Prior service cost amortization	25	25	76	76
Net periodic benefit cost	2,189	1,592	6,567	4,775
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1,213	1,101	3,640	3,303
Interest cost	1,757	2,081	5,270	6,243
Expected return on plan assets	(1,628)	(1,379)	(4,883)	(4,137)
Recognized actuarial loss	1,861	1,617	5,582	4,850
Prior service cost amortization	(49)	(91)	(148)	(273)
Net periodic benefit cost	$ 3,154	$ 3,329	$ 9,461	$ 9,986