REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Regulatory Liabilities	$ 605,000	$ 377,000
Regulatory Liabilities, Current	89,024	76,069
Regulatory Liabilities, Noncurrent	516,438	300,700
Income Taxes Payable through Future Rates
Regulatory Liabilities [Line Items]
Regulatory Liabilities	353,000	3,000
Net Cost of Removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities	180,000	270,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Regulatory Liabilities	44,000	32,000
Deferred Investment Tax Credits
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 14,000	23,000
Purchased Power and Fuel Adjustment Clause
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Regulatory Liabilities	$ 9,000	38,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 5,000	$ 11,000