PRESENTATION OF FINANCIAL INFORMATION (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) item class facility
Jointly owned facilities
Number of jointly owned facilities | facility	2
Electric plant in service	$ 883,160
Accumulated depreciation	541,588
Construction work in progress	$ 4,026
Member information
Number of classes of members | class	2
Number of members | item	43
Yampa
Jointly owned facilities
Tri-State share (as a percent)	24.00%
Electric plant in service	$ 396,111
Accumulated depreciation	243,788
Construction work in progress	$ 65
MBPP
Jointly owned facilities
Tri-State share (as a percent)	27.13%
Electric plant in service	$ 487,049
Accumulated depreciation	297,800
Construction work in progress	$ 3,961