LEASES - ADDITIONAL INFORMATION (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Lessee, Lease, Description [Line Items]
ROU asset obtained in exchange for operating lease liability	$ 5.7	$ 12.4	$ 13.3	$ 34.0
ROU asset obtained in exchange for finance lease liability	$ 0.0	$ 45.7	$ 0.0	$ 49.7
Weighted average remaining lease term, operating lease	27 years 7 months 6 days		27 years 7 months 6 days		25 years 6 months
Operating lease, discount rate	3.30%		3.30%		3.20%
Weighted average remaining lease term, finance lease	11 years 10 months 24 days		11 years 10 months 24 days		11 years 6 months
Finance lease, discount rate	2.50%		2.50%		2.50%
Solar Property | Minimum
Lessee, Lease, Description [Line Items]
Term of contract	15 years		15 years
Renewal term	5 years		5 years
Solar Property | Maximum
Lessee, Lease, Description [Line Items]
Term of contract	35 years		35 years
Renewal term	10 years		10 years
Office Building | Minimum
Lessee, Lease, Description [Line Items]
Term of contract	1 year		1 year
Office Building | Maximum
Lessee, Lease, Description [Line Items]
Term of contract	25 years		25 years
Meter License
Lessee, Lease, Description [Line Items]
Term of contract	7 years		7 years
Equipment
Lessee, Lease, Description [Line Items]
Term of contract	5 years		5 years
Storage and Capacity
Lessee, Lease, Description [Line Items]
Term of contract	50 years		50 years