REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Liabilities	$ 573,000	$ 607,000
Less Current Portion	96,017	95,094
Total Non-Current Regulatory Liabilities	477,495	512,425
Income Taxes Payable through Future Rates
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	327,000	354,000
Net Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	164,000	171,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	59,000	52,000
Deferred Investment Tax Credits
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	3,000	7,000
Over Recovered Purchased Energy Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	17,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 20,000	$ 6,000