PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2026	Dec. 31, 2025
Electric Plant
Accumulated Depreciation	$ (3,005,440)	$ (2,913,707)
Construction work in progress	197,543	143,940
Net electric plant	3,341,609	3,305,524
Electric Plant
Electric Plant
Plant In Service	6,149,506	6,075,291
Accumulated Depreciation	(3,005,440)	(2,913,707)
Net Book Value	3,144,066	3,161,584
Construction work in progress	197,543	143,940
Net electric plant	3,341,609	3,305,524
Generation plant
Electric Plant
Plant In Service	3,367,291	3,375,766
Accumulated Depreciation	(1,908,476)	(1,836,220)
Net Book Value	$ 1,458,815	$ 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	$ 2,254,630	$ 2,174,345
Accumulated Depreciation	(839,831)	(828,367)
Net Book Value	$ 1,414,799	$ 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	$ 278,372	$ 278,735
Accumulated Depreciation	(151,543)	(146,589)
Net Book Value	$ 126,829	$ 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	$ 249,213	$ 246,445
Accumulated Depreciation	(105,590)	(102,531)
Net Book Value	$ 143,623	$ 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%

Period end amount of total net PPE excluding construction work in process.

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- Definition

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

+ References

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- Definition

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

+ References

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Period end amount of construction work in progress in public utility.

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- Definition

Composite depreciation rate for public utility plants in service.

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- Definition

Period end amount of total net PPE.

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