CONCENTRATIONS OF CREDIT RISK (Details) $ in Millions	9 Months Ended
	Jun. 30, 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	$ 10.6
Net receivable amount	5.6
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	63.8
Net receivable amount	60.0
Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	30.7
Net receivable amount	$ 29.0