v3.26.1
PROPERTY, PLANT AND EQUIPMENT
 3 Months Ended
Mar. 31, 2026
Public Utilities, Property, Plant and Equipment [Abstract]  
PROPERTY, PLANT AND EQUIPMENT PROPERTY, PLANT AND EQUIPMENT
Tri-State's property, plant and equipment on its consolidated statements of financial position consists of electric plant and other plant.
ELECTRIC PLANT:  As of March 31, 2026, Tri-State's 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 %$3,363,251 $(1,861,348)$1,501,903 
Transmission plant1.17 %to1.84 %2,199,596 (834,884)1,364,712 
General plant1.20 %to11.60 %287,617 (150,384)137,233 
Other2.75 %to10.00 %246,414 (103,457)142,957 
Electric plant in service (at cost)$6,096,878 $(2,950,073)3,146,805 
Construction work in progress173,101 
Electric plant$3,319,906 

As of December 31, 2025, Tri-State's 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 %$3,375,766 $(1,836,220)$1,539,546 
Transmission plant1.17 %to1.84 %2,174,345 (828,367)1,345,978 
General plant1.20 %to11.60 %278,735 (146,589)132,146 
Other2.75 %to10.00 %246,445 (102,531)143,914 
Electric plant in service (at cost)$6,075,291 $(2,913,707)3,161,584 
Construction work in progress143,940 
Electric plant$3,305,524 
JOINTLY OWNED FACILITIES:  Tri-State's share in each jointly owned facility is as follows as of March 31, 2026 (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 %$392,520 $380,700 $— 
MBPP - Laramie River Station28.50 %543,600 363,581 4,588 
Total$936,120 $744,281 $4,588 
OTHER PLANT:  Other plant consists of mine assets (discussed below) and non‑utility assets which consist of facilities not in service, land and irrigation equipment. The depreciation basis for all other plant assets is the straight-line method.
Tri-State owns 100 percent of Elk Ridge Mining and Reclamation, LLC (“Elk Ridge”), organized for the purpose of acquiring coal reserves and supplying coal for electric generation. Elk Ridge is the owner of the Colowyo Mine, a surface coal mine near Craig, Colorado, and the New Horizon Mine near Nucla, Colorado. The New Horizon Mine is in post-reclamation monitoring and no longer produces coal. The Colowyo Mine is in final reclamation and no longer produces coal. The expenses related to the Colowyo Mine coal used by Tri-State are included in fuel expense on Tri-State's consolidated statements of operations.
Other plant assets are as follows (dollars in thousands):
March 31,
2026		December 31,
2025
Colowyo Mine assets$247,985 $297,636 
New Horizon Mine assets6,287 6,287 
Accumulated depreciation and depletion(195,959)(232,575)
Net mine assets58,313 71,348 
Non-utility assets549,158 549,487 
Accumulated depreciation(527,909)(527,902)
Net non-utility assets21,249 21,585 
Net other plant$79,562 $92,933