CONCENTRATIONS OF CREDIT RISK (Details) $ in Millions	Sep. 30, 2018 USD ($) counterparty
Concentration Risk [Line Items]
Number of large counterparties for which credit risk is disclosed | counterparty	5
Energy Producers And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	$ 11.2
Net receivable amount	3.7
Utility Companies And Their Affiliates
Concentration Risk [Line Items]
Accounts receivable	96.1
Net receivable amount	89.7
Five Largest Counterparties
Concentration Risk [Line Items]
Accounts receivable	51.3
Net receivable amount	$ 50.6