REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 309,000	$ 330,000
Less Current Portion	112,782	147,389
Total Noncurrent Regulatory Assets	195,953	182,997
Pension and Other Postretirement Benefits (Note 7)
Regulatory Assets [Line Items]
Total Regulatory Assets	106,000	107,000
Early Generation Retirement Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 47,000	48,000
Derivatives (Note 8)
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	6 years
Total Regulatory Assets	$ 40,000	26,000
Lost Fixed Cost Recovery
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 33,000	35,000
Property Tax Deferrals
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 30,000	30,000
Final Mine Reclamation and Retiree Healthcare Costs
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	16 years
Total Regulatory Assets	$ 21,000	6,000
Under-Recovered Purchased Energy Costs
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 7,000	55,000
Income Taxes Recoverable through Future Rates
Regulatory Assets [Line Items]
Total Regulatory Assets	6,000	6,000
Unamortized Loss on Reacquired Debt
Regulatory Assets [Line Items]
Total Regulatory Assets	5,000	5,000
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 14,000	$ 12,000