REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative and Regulatory Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) site asset	Dec. 31, 2016 USD ($)
Regulatory Liabilities [Line Items]
Regulatory Assets	$ 477,457	$ 410,746
Total Regulatory Liabilities	23,485	49,121
Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	23,485	28,226
Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Regulatory Assets	15,200
Total Regulatory Liabilities	0	17,800
Societal Benefit Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 0	$ 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