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PROPERTY, PLANT AND EQUIPMENT (Tables)
 9 Months Ended
Sep. 30, 2024
Public Utilities, Property, Plant and Equipment [Abstract]  
Schedule of Investment in Electric Plant
ELECTRIC PLANT:  As of September 30, 2024, our investment in electric plant and the related annual rates of depreciation or amortization calculated using the straight‑line method are as follows (dollars in thousands):
Annual Depreciation RatePlant In
Service		Accumulated
Depreciation		Net Book
Value
Generation plant1.14 %to4.14 %$3,098,909 $(1,721,383)$1,377,526 
Transmission plant1.17 %to1.84 %1,929,078 (706,983)1,222,095 
General plant1.20 %to11.60 %430,202 (279,264)150,938 
Other2.75 %to10.00 %240,705 (98,088)142,617 
Electric plant in service (at cost)$5,698,894 $(2,805,718)2,893,176 
Construction work in progress273,339 
Electric plant$3,166,515 
As of December 31, 2023, our investment in electric plant and the related annual rates of depreciation or amortization calculated using the straight‑line method are as follows (dollars in thousands):
Annual Depreciation RatePlant In
Service		Accumulated
Depreciation		Net Book
Value
Generation plant0.89 %to6.27 %$3,082,133 $(1,669,941)$1,412,192 
Transmission plant1.11 %to2.09 %1,983,629 (708,412)1,275,217 
General plant1.46 %to9.53 %410,856 (266,013)144,843 
Other2.75 %to10.00 %246,061 (95,558)150,503 
Electric plant in service (at cost)$5,722,679 $(2,739,924)2,982,755 
Construction work in progress163,954 
Electric plant$3,146,709 
Schedule of Jointly Owned Facilities
JOINTLY OWNED FACILITIES:  Our share in each jointly owned facility is as follows as of September 30, 2024 (these electric plant in service, accumulated depreciation and construction work in progress amounts are included in the electric plant table above) (dollars in thousands):
Tri-State
Share		Electric
Plant in
Service		Accumulated
Depreciation		Construction
Work In
Progress
Yampa Project - Craig Generating Station Units 1 and 224.00 %$392,510 $365,379 $— 
MBPP - Laramie River Station28.50 %545,456 351,886 8,928 
Total$937,966 $717,265 $8,928 
Schedule of Other Plant Assets (Excluding Utility Assets)
Other plant assets are as follows (dollars in thousands):
September 30,
2024		December 31,
2023
Colowyo Mine assets$408,359 $396,441 
New Horizon Mine assets6,287 6,448 
Accumulated depreciation and depletion(216,142)(184,239)
Net mine assets198,504 218,650 
Non-utility assets549,225 549,430 
Accumulated depreciation(527,610)(527,658)
Net non-utility assets21,615 21,772 
Net other plant$220,119 $240,422