VARIABLE INTEREST ENTITIES - Summary (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Variable Interest Entity [Line Items]
Net electric plant	$ 3,157,823		$ 3,157,823		$ 3,146,709
Noncontrolling interest	129,738		129,738		134,269
Long-term debt	2,706,512		2,706,512		2,896,506
Accrued interest	22,545		22,545		24,549
Interest	42,614	$ 41,495	85,108	$ 81,276
Variable Interest Entity, Primary Beneficiary | Reportable Legal Entities
Variable Interest Entity [Line Items]
Net electric plant	694,790		694,790		703,859
Noncontrolling interest	129,737		129,737		134,269
Long-term debt	173,091		173,091		206,027
Accrued interest	4,997		4,997		$ 5,968
Depreciation, amortization and depletion	4,535	4,535	9,069	9,069
Interest	$ 2,852	$ 3,401	$ 5,885	$ 7,068