PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2026	Jun. 30, 2025	Dec. 31, 2025
Electric Plant
Accumulated Depreciation	$ (2,950,073)		$ (2,913,707)
Construction work in progress	173,101		143,940
Net electric plant	3,319,906		3,305,524
Electric Plant
Electric Plant
Plant In Service	6,096,878		6,075,291
Accumulated Depreciation	(2,950,073)		(2,913,707)
Net Book Value	3,146,805		3,161,584
Construction work in progress	173,101		143,940
Net electric plant	3,319,906		3,305,524
Generation plant
Electric Plant
Plant In Service	3,363,251		3,375,766
Accumulated Depreciation	(1,861,348)		(1,836,220)
Net Book Value	$ 1,501,903		1,539,546
Generation plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.14%	1.14%
Generation plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	4.14%	4.14%
Transmission plant
Electric Plant
Plant In Service	$ 2,199,596		2,174,345
Accumulated Depreciation	(834,884)		(828,367)
Net Book Value	$ 1,364,712		1,345,978
Transmission plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.17%	1.17%
Transmission plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	1.84%	1.84%
General plant
Electric Plant
Plant In Service	$ 287,617		278,735
Accumulated Depreciation	(150,384)		(146,589)
Net Book Value	$ 137,233		132,146
General plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	1.20%	1.20%
General plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	11.60%	11.60%
Other
Electric Plant
Plant In Service	$ 246,414		246,445
Accumulated Depreciation	(103,457)		(102,531)
Net Book Value	$ 142,957		$ 143,914
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%

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Period end amount of total gross PPE, excluding construction work in progress.

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Period end book value of accumulated depreciation on property, plant and equipment (PPE) that is owned by the regulated operations of the public utility.

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Composite depreciation rate for public utility plants in service.

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Period end amount of total net PPE.

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