PROPERTY, PLANT AND EQUIPMENT - Revised SIP (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) MW	Dec. 31, 2016 USD ($)
Jointly owned facilities
Net book value	$ 3,569,391	$ 3,569,391	$ 3,533,139
Craig Generating Station Unit 1
Jointly owned facilities
Generating unit capacity (in megawatts) | MW		102
Tri-State share (as a percent)	24.00%	24.00%
Net book value	$ 26,900	$ 26,900
Increase (decrease) in depreciation expense		2,900
Nucla Generating Station
Jointly owned facilities
Net book value	55,200	55,200
Increase (decrease) in depreciation expense		8,800
Transmission and general plants | Adopted Depreciation Rates
Jointly owned facilities
Increase (decrease) in depreciation expense		$ (14,200)
Generation plant | Adopted Depreciation Rates
Jointly owned facilities
Increase (decrease) in depreciation expense	$ 700