REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Liabilities	$ 483,000	$ 489,000
Less Current Portion	91,737	92,740
Total Noncurrent Regulatory Liabilities	391,492	396,061
Income Taxes Payable through Future Rates
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	225,000	229,000
Net Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	132,000	130,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 76,000	77,000
Derivatives (Note 8)
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	6 years
Total Regulatory Liabilities	$ 28,000	28,000
Demand Side Management
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Liabilities	$ 10,000	9,000
Deferred Investment Tax Credits
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	6,000	6,000
Pension and Other Postretirement Benefits (Note 7)
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 4,000	4,000
Transmission Revenue Subject to Refund - FERC
Regulatory Liabilities [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Liabilities	$ 2,000	5,000
Other Regulatory Assets
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 0	$ 1,000