NATURE OF OPERATIONS AND FINANCIAL STATEMENT PRESENTATION (Nature of Operations) (Details) customer in Thousands, $ in Thousands		9 Months Ended
	Jan. 01, 2018 USD ($)	Sep. 30, 2018 USD ($) mi² customer
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of retail customers | customer		425
Area in which company generates transmits and distributes electricity to retail electric customers (square mile) | mi²		1,155
AOCI Attributable to Parent
Adoption of ASU, Cumulative Effect Adjustment	$ (1,000)	$ (878)
Retained Earnings
Adoption of ASU, Cumulative Effect Adjustment	$ 1,000	$ 878