PROPERTY, PLANT AND EQUIPMENT (Tables)

3 Months Ended

Mar. 31, 2026

Public Utilities, Property, Plant and Equipment [Abstract]

Schedule of Investment in Electric 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 Rate					Plant In Service		Accumulated Depreciation		Net Book Value
Generation plant	1.14	%	to	4.14	%	$	3,363,251	$	(1,861,348)	$	1,501,903
Transmission plant	1.17	%	to	1.84	%	2,199,596		(834,884)		1,364,712
General plant	1.20	%	to	11.60	%	287,617		(150,384)		137,233
Other	2.75	%	to	10.00	%	246,414		(103,457)		142,957
Electric plant in service (at cost)						$	6,096,878	$	(2,950,073)	3,146,805
Construction work in progress										173,101
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 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 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 2	24.00	%	$	392,520	$	380,700	$	—
MBPP - Laramie River Station	28.50	%	543,600		363,581		4,588
Total			$	936,120	$	744,281	$	4,588

Schedule of Other Plant Assets (Excluding Utility Assets)

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 assets	6,287		6,287

Accumulated depreciation and depletion	(195,959)		(232,575)
Net mine assets	58,313		71,348
Non-utility assets	549,158		549,487
Accumulated depreciation	(527,909)		(527,902)
Net non-utility assets	21,249		21,585
Net other plant	$	79,562	$	92,933