EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2,183	$ 2,055	$ 4,365	$ 4,111
Interest cost	2,278	2,646	4,556	5,293
Expected return on plan assets	(4,851)	(5,232)	(10,075)	(10,289)
Recognized actuarial loss	2,861	2,466	5,723	5,212
Prior service cost (credit) amortization	26	26	51	51
Net periodic benefit cost	2,497	1,961	4,620	4,378
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1,211	1,072	2,422	2,427
Interest cost	1,517	1,509	3,035	3,513
Expected return on plan assets	(1,565)	(1,695)	(3,248)	(3,255)
Recognized actuarial loss	1,978	930	3,955	3,721
Prior service cost (credit) amortization	(45)	(50)	(90)	(99)
Net periodic benefit cost	$ 3,096	$ 1,766	$ 6,074	$ 6,307