REGULATORY MATTERS (Regulatory Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Regulatory Liabilities [Line Items]
Remaining recovery period (years)	1 year
Total Regulatory Liabilities	$ 463,000	$ 541,000
Less Current Portion	111,356	151,189
Total Non-Current Regulatory Liabilities	352,226	390,164
Income Taxes Payable through Future Rates
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	268,000	298,000
Net Cost of Removal
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	73,000	125,000
Renewable Energy Standard
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	66,000	63,000
Derivatives
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 19,000	4,000
Derivatives | Derivatives
Regulatory Liabilities [Line Items]
Remaining recovery period (years)	8 years
Transmission Revenue Subject to Refund—FERC
Regulatory Liabilities [Line Items]
Remaining recovery period (years)	1 year
Total Regulatory Liabilities	$ 15,000	15,000
Demand Side Management
Regulatory Liabilities [Line Items]
Remaining recovery period (years)	1 year
Total Regulatory Liabilities	$ 12,000	6,000
Transmission Cost Adjustor
Regulatory Liabilities [Line Items]
Remaining recovery period (years)	1 year
Total Regulatory Liabilities	$ 9,000	0
Tax Expense Adjustor Mechanism
Regulatory Liabilities [Line Items]
Remaining recovery period (years)	1 year
Total Regulatory Liabilities	$ 0	29,000
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 1,000	$ 1,000