COMMITMENTS AND CONTINGENCIES - Electric Power Purchase Agreements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) contract MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long-term Transmission Agreement Dispute
Legal
Damages sought | $	$ 6,900
Basin Electric Power Cooperative
Electric Power Purchase Agreements
Number of contracts	2
Costs under electric power purchase agreements | $	$ 145,008	$ 149,246	$ 152,977
WAPA
Electric Power Purchase Agreements
Number of contracts	5
Costs under electric power purchase agreements | $	$ 72,504	72,757	78,781
Other Renewables
Electric Power Purchase Agreements
Megawatts purchased | MW	1,498
Costs under electric power purchase agreements | $	$ 63,677	$ 62,721	$ 53,362
Wind-based Power
Electric Power Purchase Agreements
Megawatts purchased | MW	671
Solar-based Power
Electric Power Purchase Agreements
Megawatts purchased | MW	800
Loveland Area Projects | WAPA
Electric Power Purchase Agreements
Number of contracts	2
Loveland Area Projects, Contract Terminating 2024 | WAPA
Electric Power Purchase Agreements
Number of contracts	1
Loveland Area Projects, Contract Terminating 2054 | WAPA
Electric Power Purchase Agreements
Number of contracts	1
Salt Lake City Area Integrated Projects | WAPA
Electric Power Purchase Agreements
Number of contracts	3
Salt Lake City Area Integrated Projects, Contracts Terminating 2024 | WAPA
Electric Power Purchase Agreements
Number of contracts	2
Salt Lake City Area Integrated Projects, Contract Terminating 2057 | WAPA
Electric Power Purchase Agreements
Number of contracts	1
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