CONCENTRATIONS OF CREDIT RISK (Details) $ in Millions	6 Months Ended
	Mar. 31, 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	0
Number of counterparties with non-investment grade rating | counterparty	3
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 15.6
Net receivable amount	9.6
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	30.7
Net receivable amount	29.4
Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	16.1
Net receivable amount	$ 14.9