PRESENTATION OF FINANCIAL INFORMATION (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) item class facility
Jointly owned facilities
Number of jointly owned facilities | facility	2
Electric plant in service	$ 885,213
Accumulated depreciation	549,105
Construction work in progress	$ 4,069
Member information
Number of classes of members | class	3
Yampa
Jointly owned facilities
Tri-State share (as a percent)	24.00%
Electric plant in service	$ 396,334
Accumulated depreciation	249,757
Construction work in progress	$ 201
MBPP
Jointly owned facilities
Tri-State share (as a percent)	27.13%
Electric plant in service	$ 488,879
Accumulated depreciation	299,348
Construction work in progress	$ 3,868
Class A Members
Member information
Number of members | item	43
Class B Members
Member information
Number of classes of members | class	0
Non Utility Members
Member information
Number of members | item	3