PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Electric Plant
Plant In Service	$ 5,722,679	$ 5,659,423
Accumulated Depreciation	(2,739,924)	(2,392,363)
Construction work in progress	163,954	81,555
Net electric plant	3,146,709	3,348,615
Electric Plant
Electric Plant
Plant In Service	5,722,679	5,659,423
Accumulated Depreciation	(2,739,924)	(2,392,363)
Net Book Value	2,982,755	3,267,060
Construction work in progress	163,954	81,555
Net electric plant	3,146,709	3,348,615
Generation plant
Electric Plant
Plant In Service	3,082,133	3,048,918
Accumulated Depreciation	(1,669,941)	(1,353,042)
Net Book Value	$ 1,412,192	$ 1,695,876
Generation plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	0.89%	0.89%
Generation plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	6.27%	6.27%
Transmission plant
Electric Plant
Plant In Service	$ 1,983,629	$ 1,956,378
Accumulated Depreciation	(708,412)	(690,517)
Net Book Value	$ 1,275,217	$ 1,265,861
Transmission plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.11%	1.11%
Transmission plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	2.09%	2.09%
General plant
Electric Plant
Plant In Service	$ 410,856	$ 412,879
Accumulated Depreciation	(266,013)	(257,761)
Net Book Value	$ 144,843	$ 155,118
General plant | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.46%	1.46%
General plant | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	9.53%	9.53%
Other
Electric Plant
Plant In Service	$ 246,061	$ 241,248
Accumulated Depreciation	(95,558)	(91,043)
Net Book Value	$ 150,503	$ 150,205
Other | Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	2.75%	2.75%
Other | Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	10.00%	10.00%