REGULATORY ASSETS AND REGULATORY LIABILITIES - NARRATIVE AND REGULATORY LIABILITIES (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) site asset	Dec. 31, 2016 USD ($)
Regulatory Assets and Liabilities Disclosure [Abstract]
Number of sites under remediation | site	2
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 37,308	$ 49,121
Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	27,143	28,226
Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	10,087	17,800
Societal Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 78	$ 3,095
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