REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Liabilities	$ 575,000	$ 573,000
Less Current Portion	99,593	96,017
Total Non-Current Regulatory Liabilities	475,232	477,495
Income Taxes Payable through Future Rates
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	325,000	327,000
Net Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	159,000	164,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	59,000	59,000
Deferred Investment Tax Credits
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	3,000	3,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 29,000	$ 20,000