REGULATORY ASSETS AND LIABILITIES REGULATORY ASSETS AND LIABILITIES (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) site sites asset	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items]
Number of sites for environmental cleanup | sites	12
Regulatory assets	$ 380,364		$ 323,434
Regulatory Liabilities [Line Items]
Regulatory liabilities	60,010		42,841
Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Regulatory liabilities	18,631		0
Energy Efficiency Tracker
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 2,068		0
Environmental Restoration Costs
Regulatory Assets [Line Items]
Number of regulatory assets associated with environmental costs | asset	2
Number of sites for environmental cleanup | site	12
Recovery period for 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