EMPLOYEE BENEFIT PLANS - ADDITIONAL INFORMATION (Details)	6 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) plan	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of plans | plan	2
Required length of service	1 year
Basis period	60 months
Employer contributions	$ 0	$ 0
OPEB
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			$ 7,926,000
OPEB | Minimum
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions	5,000,000
OPEB | Maximum
Defined Benefit Plan Disclosure [Line Items]
Estimated future employer contributions	$ 10,000,000