EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2,182	$ 2,056
Interest cost	2,278	2,647
Expected return on plan assets	(5,224)	(5,057)
Recognized actuarial loss	2,862	2,746
Prior service cost (credit) amortization	25	25
Net periodic benefit cost	2,123	2,417
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1,211	1,355
Interest cost	1,518	2,004
Expected return on plan assets	(1,683)	(1,560)
Recognized actuarial loss	1,977	2,791
Prior service cost (credit) amortization	(45)	(49)
Net periodic benefit cost	$ 2,978	$ 4,541