COMMITMENTS AND CONTINGENCIES - Electric Power Purchase Agreements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) contract MW	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Basin Electric Power Cooperative
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	2
Costs under electric power purchase agreements | $	$ 142,456	$ 148,146	$ 146,532
Basin Electric Power Cooperative | Eastern Interconnection
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	1
Basin Electric Power Cooperative | Western Interconnection
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	1
WAPA
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	3
Costs under electric power purchase agreements | $	$ 74,899	67,791	70,107
WAPA | Loveland Area Projects, Contract Terminating 2024
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	1
WAPA | Loveland Area Projects, Contract Terminating 2054
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	1
WAPA | Salt Lake City Area Integrated Projects, Contracts Terminating 2024
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	2
WAPA | Salt Lake City Area Integrated Projects, Contract Terminating 2057
Long-Term Contract for Purchase of Electric Power [Line Items]
Number of contracts	1
Other Renewables
Long-Term Contract for Purchase of Electric Power [Line Items]
Megawatts purchased | MW	1,381
Costs under electric power purchase agreements | $	$ 78,177	$ 85,601	$ 71,565
Wind-based Power
Long-Term Contract for Purchase of Electric Power [Line Items]
Megawatts purchased | MW	674
Solar-based Power
Long-Term Contract for Purchase of Electric Power [Line Items]
Megawatts purchased | MW	680