REGULATORY ASSETS & REGULATORY LIABILITIES REGULATORY ASSETS & REGULATORY LIABILITIES (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) site asset	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items]
Regulatory assets	$ 356,962		$ 323,434
Regulatory Liabilities [Line Items]
Regulatory liabilities	55,373		42,841
Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Regulatory liabilities	11,241		0
Energy Efficiency Tracker
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 2,384		0
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
Deferred Revenues - Net
Regulatory Assets [Line Items]
Regulatory assets			2,700
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Regulatory assets	$ 0		$ 496
Regulatory Liabilities [Line Items]
Decrease in amount of regulatory costs approved		$ 7,900