REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Regulatory Liabilities	$ 607,000	$ 605,000
Regulatory Liabilities, Current	95,094	89,024
Regulatory Liabilities, Noncurrent	512,425	516,438
Income Taxes Payable through Future Rates
Regulatory Liabilities [Line Items]
Regulatory Liabilities	354,000	353,000
Net Cost of Removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities	171,000	180,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 52,000	44,000
Purchased Power and Fuel Adjustment Clause
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Regulatory Liabilities	$ 17,000	9,000	$ 38,000
Deferred Investment Tax Credits
Regulatory Liabilities [Line Items]
Regulatory Liabilities	7,000	14,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 6,000	$ 5,000