REGULATORY MATTERS (Regulatory Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 341,000	$ 370,000
Less Current Portion	163,327	185,034
Total Non-Current Regulatory Assets	177,814	184,894
Under Recovered Purchased Energy Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 93,000	124,000
Remaining Recovery Period (years)	1 year
Pension and Other Postretirement Benefits
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 87,000	90,000
Early Generation Retirement Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 55,000	58,000
Lost Fixed Cost Recovery
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 33,000	25,000
Property Tax Deferrals
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	1 year
Total Regulatory Assets	$ 30,000	29,000
Final Mine Reclamation and Retiree Healthcare Costs
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	6 years
Total Regulatory Assets	$ 11,000	11,000
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
Derivatives
Regulatory Assets [Line Items]
Remaining Recovery Period (years)	7 years
Total Regulatory Assets	$ 3,000	3,000
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 18,000	$ 19,000