REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative and Regulatory Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) site asset	Dec. 31, 2017 USD ($)
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 300,358	$ 287,105
Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Increase in Regulatory Assets	37,500
Tax Reform
Regulatory Liabilities [Line Items]
Increase in Regulatory Assets	13,000
Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	23,527	23,295
Energy Efficiency Tracker
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 276,831	$ 263,810
Environmental restoration costs
Regulatory Assets [Line Items]
Number of regulatory assets | asset	2
Number of sites for environmental cleanup | site	12
Original recovery period of expenditures	7 years