PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Electric Plant
Accumulated Depreciation	$ (2,772,281)	$ (2,739,924)
Construction work in progress	256,301	163,954
Net electric plant	3,157,823	3,146,709
Electric Plant
Electric Plant
Plant In Service	5,673,803	5,722,679
Accumulated Depreciation	(2,772,281)	(2,739,924)
Net Book Value	2,901,522	2,982,755
Construction work in progress	256,301	163,954
Net electric plant	3,157,823	3,146,709
Generation plant
Electric Plant
Plant In Service	3,089,810	3,082,133
Accumulated Depreciation	(1,699,258)	(1,669,941)
Net Book Value	$ 1,390,552	$ 1,412,192
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	$ 1,895,240	$ 1,983,629
Accumulated Depreciation	(701,422)	(708,412)
Net Book Value	$ 1,193,818	$ 1,275,217
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	$ 448,016	$ 410,856
Accumulated Depreciation	(274,586)	(266,013)
Net Book Value	$ 173,430	$ 144,843
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	$ 240,737	$ 246,061
Accumulated Depreciation	(97,015)	(95,558)
Net Book Value	$ 143,722	$ 150,503
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%