VARIABLE INTEREST ENTITIES - Summary (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Variable Interest Entity [Line Items]
Net electric plant	$ 3,166,515		$ 3,166,515		$ 3,146,709
Noncontrolling interest	130,453		130,453		134,269
Long-term debt	2,696,693		2,696,693		2,896,506
Accrued interest	42,930		42,930		24,549
Interest	40,308	$ 44,320	125,416	$ 125,596
Variable Interest Entity, Primary Beneficiary | Reportable Legal Entities
Variable Interest Entity [Line Items]
Net electric plant	686,514		686,514		703,859
Noncontrolling interest	130,453		130,453		134,269
Long-term debt	172,942		172,942		206,027
Accrued interest	1,999		1,999		$ 5,968
Depreciation, amortization and depletion	8,276	4,535	17,345	13,603
Interest	$ 2,849	$ 3,397	$ 8,734	$ 10,465