REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Regulatory Liabilities	$ 377,000	$ 360,000
Regulatory Liabilities, Current	76,069	53,077
Regulatory Liabilities, Noncurrent	300,700	307,286
Net Cost of Removal for Interim Retirements (6)
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 270,000	264,000
Purchased Power and Fuel Adjustment Clause
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Regulatory Liabilities	$ 38,000	18,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Regulatory Liabilities	32,000	25,000
Deferred Investment Tax Credits (7)
Regulatory Liabilities [Line Items]
Regulatory Liabilities	23,000	32,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 14,000	$ 21,000