PRESENTATION OF FINANCIAL INFORMATION (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) item class facility
Jointly owned facilities
Number of jointly owned facilities | facility	2
Electric plant in service	$ 884,246
Accumulated depreciation	551,365
Construction work in progress	$ 5,409
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	$ 395,099
Accumulated depreciation	249,804
Construction work in progress	$ 334
MBPP
Jointly owned facilities
Tri-State share (as a percent)	27.13%
Electric plant in service	$ 489,147
Accumulated depreciation	301,561
Construction work in progress	$ 5,075
Class A Members
Member information
Number of members | item	42
Class B Members
Member information
Number of members | item	0
Non-Utility Members
Member information
Number of members | item	3