REGULATORY ASSETS & REGULATORY LIABILITIES - LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 287,105	$ 49,121
Regulatory assets	469,224	410,746
Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		17,800
Regulatory assets	16,800
Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	23,295	28,226
Deferred Revenue - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	17,800
Societal Benefit Costs Payable
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	3,095
Energy Efficiency Tracker
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 263,810	$ 0