PROPERTY, PLANT AND EQUIPMENT (Tables)

9 Months Ended

Sep. 30, 2026

Public Utilities, Property, Plant and Equipment [Abstract]

Schedule of Investment in Electric Plant

ELECTRIC PLANT: As of September 30, 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 Rate					Plant In Service		Accumulated Depreciation		Net Book Value
Generation plant	1.14	%	to	4.14	%	$	3,367,291	$	(1,908,476)	$	1,458,815
Transmission plant	1.17	%	to	1.84	%	2,254,630		(839,831)		1,414,799
General plant	1.20	%	to	11.60	%	278,372		(151,543)		126,829
Other	2.75	%	to	10.00	%	249,213		(105,590)		143,623
Electric plant in service (at cost)						$	6,149,506	$	(3,005,440)	3,144,066
Construction work in progress										197,543
Electric plant										$	3,341,609

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 Rate					Plant In Service		Accumulated Depreciation		Net Book Value
Generation plant	1.14	%	to	4.14	%	$	3,375,766	$	(1,836,220)	$	1,539,546
Transmission plant	1.17	%	to	1.84	%	2,174,345		(828,367)		1,345,978
General plant	1.20	%	to	11.60	%	278,735		(146,589)		132,146
Other	2.75	%	to	10.00	%	246,445		(102,531)		143,914
Electric plant in service (at cost)						$	6,075,291	$	(2,913,707)	3,161,584
Construction work in progress										143,940
Electric plant										$	3,305,524

Schedule of Jointly Owned Facilities

JOINTLY OWNED FACILITIES: Tri-State's share in each jointly owned facility is as follows as of September 30, 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 2	24.00	%	$	392,523	$	383,905	$	—
MBPP - Laramie River Station	28.50	%	546,356		367,881		3,616
Total			$	938,879	$	751,786	$	3,616

Schedule of Other Plant Assets (Excluding Utility Assets)

Other plant assets are as follows (dollars in thousands):


	September 30, 2026		December 31, 2025
Colowyo Mine assets	$	247,255	$	297,636
New Horizon Mine assets	6,287		6,287

Accumulated depreciation and depletion	(197,058)		(232,575)
Net mine assets	56,484		71,348
Non-utility assets	551,402		549,487
Accumulated depreciation	(528,030)		(527,902)
Net non-utility assets	23,372		21,585
Net other plant	$	79,856	$	92,933