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PROPERTY, PLANT AND EQUIPMENT (Tables)
 12 Months Ended
Dec. 31, 2023
Public Utilities, Property, Plant and Equipment [Abstract]  
Schedule of Investment in Electric Plant
ELECTRIC PLANT:  At 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 
At December 31, 2022, 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 %$3,048,918 $(1,353,042)$1,695,876 
Transmission plant1.11 %to2.09 %1,956,378 (690,517)1,265,861 
General plant1.46 %to9.53 %412,879 (257,761)155,118 
Other2.75 %to10.00 %241,248 (91,043)150,205 
Electric plant in service (at cost)$5,659,423 $(2,392,363)3,267,060 
Construction work in progress81,555 
Electric plant$3,348,615 
Schedule of Jointly Owned Facilities
JOINTLY OWNED FACILITIES:  Our share in each jointly owned facility is as follows as of December 31, 2023 (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 $358,839 $23 
MBPP - Laramie River Station28.50 %540,365 346,959 2,114 
Total$932,875 $705,798 $2,137 
Schedule of Other Plant Assets (Excluding Utility Assets)
Other plant assets are as follows (dollars in thousands):
December 31,
2023		December 31,
2022
Colowyo Mine assets$396,441 $386,898 
New Horizon Mine assets6,448 5,995 
Accumulated depreciation and depletion(184,239)(155,653)
Net mine assets218,650 237,240 
Non-utility assets549,430 561,251 
Accumulated depreciation(527,658)(539,121)
Net non-utility assets21,772 22,130 
Net other plant$240,422 $259,370