CONCENTRATIONS OF CREDIT RISK (Details) $ in Millions	6 Months Ended
	Mar. 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	4
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 15.1
Net receivable amount	8.8
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	53.4
Net receivable amount	50.0
Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	30.0
Net receivable amount	$ 27.7