CONCENTRATIONS OF CREDIT RISK (Details) $ in Millions	9 Months Ended
	Jun. 30, 2018 USD ($) subsegment counterparty
Concentration Risk [Line Items]
Number of groups with potential to affect overall exposure | subsegment	3
Number of large counterparties for which credit risk is disclosed | counterparty	5
Number of counterparties with investment-grade rating | counterparty	5
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 6.5
Net receivable amount	4.2
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	53.4
Net receivable amount	50.5
Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	$ 17.7