NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Additional Information) (Details) customer in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) mi² customer	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of retail customers | customer	425
Area in which subsidiary generates transmits and distributes electricity to retail electric customers (sq miles) | mi²	1,155
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	$ 138,114		$ 37,701	$ 36,000
Total Cash, Cash Equivalents, and Restricted Cash	152,747		49,501	43,325	$ 59,867
Investments and Other Property
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash	14,000		11,000	7,000
Current Assets, Other
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash	$ 1,000		$ 1,000	$ 0
Subsequent Event | ASU 2016-02
Restricted Cash and Cash Equivalents Items [Line Items]
Operating lease liabilities		$ 8,000
Operating lease right-of-use assets		$ 8,000