COMMITMENTS AND CONTINGENCIES - Electric Power Purchase Agreements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) state contract MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Basin Electric Power Cooperative
Electric Power Purchase Agreements
Number of contracts	2
Costs under electric power purchase agreements | $	$ 152,977	$ 145,557	$ 127,500
WAPA
Electric Power Purchase Agreements
Number of contracts	3
Costs under electric power purchase agreements | $	$ 78,781	82,575	89,986
Wind and Solar
Electric Power Purchase Agreements
Megawatts purchased | MW	477
Costs under electric power purchase agreements | $	$ 53,362	$ 42,292	$ 29,308
Wind-based Power
Electric Power Purchase Agreements
Megawatts purchased | MW	367
Solar-based Power
Electric Power Purchase Agreements
Megawatts purchased | MW	85
Loveland Area Project | WAPA
Electric Power Purchase Agreements
Number of contracts	1
Salt Lake City Area Integrated Projects | WAPA
Electric Power Purchase Agreements
Number of contracts	2
Eastern Interconnection | Basin Electric Power Cooperative
Electric Power Purchase Agreements
Number of contracts	1
Western Interconnection | Basin Electric Power Cooperative
Electric Power Purchase Agreements
Number of contracts	1
WAPA
Electric Power Purchase Agreements
Number of states in which entity operates | state	15