REGULATORY MATTERS (Federal Tax Legislation) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	May 30, 2018	Apr. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Refund to customer based on the ACC refund order		$ 27.5
Excess deferred income taxes, to be refunded to the customer		$ 9.0
Regulated operating revenue				$ 1.0
Regulatory liabilities			$ 604.0	604.0	$ 605.0
Decrease in transmission rate (percentage)	5.30%
Revenue Refund
Regulatory Liabilities [Line Items]
Regulated operating revenue			12.0	29.0
Regulatory liabilities			$ 1.0	$ 1.0