VARIABLE INTEREST ENTITIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Variable Interest Entity [Line Items]
Net electric plant	$ 3,146,709	$ 3,348,615
Noncontrolling interest	134,269	126,180
Long-term debt	2,896,506	2,869,963
Accrued interest	$ 24,549	25,431
Springerville Partnership
Variable Interest Entity [Line Items]
Equity interest (as a percent)	51.00%
General partner interest (as a percent)	1.00%
Variable Interest Entity, Primary Beneficiary | Springerville Unit 3 Lease
Variable Interest Entity [Line Items]
Proportion of financial and other support provided (as a percent)	100.00%
Variable Interest Entity, Primary Beneficiary | Springerville Unit 3 Holding LLC
Variable Interest Entity [Line Items]
Ownership interest held by parent (as a percent)	100.00%
Variable Interest Entity, Primary Beneficiary | Reportable Legal Entities
Variable Interest Entity [Line Items]
Net electric plant	$ 703,859	721,997
Noncontrolling interest	134,269	126,180
Long-term debt	206,027	254,876
Accrued interest	5,968	7,400
Depreciation, amortization and depletion	18,138	18,138	$ 18,138
Interest	$ 13,859	$ 17,064	$ 20,038
Other Limited Partners | Springerville Partnership
Variable Interest Entity [Line Items]
Ownership interest held by noncontrolling interest (as a percent)	49.00%