EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2,055	$ 1,846	$ 4,111	$ 3,691
Interest cost	2,646	3,043	5,293	6,086
Expected return on plan assets	(5,232)	(4,764)	(10,289)	(9,527)
Recognized actuarial loss	2,466	1,441	5,212	2,882
Prior service cost amortization	26	26	51	51
Net periodic benefit cost	1,961	1,592	4,378	3,183
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1,072	1,101	2,427	2,202	$ 4,404
Interest cost	1,509	2,081	3,513	4,162	$ 8,324
Expected return on plan assets	(1,695)	(1,379)	(3,255)	(2,758)
Recognized actuarial loss	930	1,616	3,721	3,233
Prior service cost amortization	(50)	(91)	(99)	(182)
Net periodic benefit cost	$ 1,766	$ 3,328	$ 6,307	$ 6,657