CONCENTRATIONS OF CREDIT RISK (Details) $ in Millions	3 Months Ended
	Dec. 31, 2017 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	3
Number of counterparties with non-investment grade rating | counterparty	2
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 19.4
Net receivable amount	15.3
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	47.5
Net receivable amount	45.4
Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	26.7
Net receivable amount	$ 23.9