UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Narrative) (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) MWh generating_units	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) MWh generating_units	May 31, 2021 MWh
Public Utility, Property, Plant and Equipment [Line Items]
Plant in service	$ 7,073,292	$ 7,797,935	$ 7,073,292
Springerville Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Percentage of interest purchased			32.20%
Purchased interest	$ 68,000
Springerville Common Facilities | Tri-State
Public Utility, Property, Plant and Equipment [Line Items]
Percentage of interest purchased	14.00%
Purchased interest	$ 30,000
Springerville Common Facilities | Tri-State | Option to Buy or Continue to Make Payments
Public Utility, Property, Plant and Equipment [Line Items]
Percentage of interest purchased		14.00%
Purchased interest		$ 30,000
Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Renewable nominal generation capacity, including PPAs (MWs) | MWh				700
Oso Grande
Public Utility, Property, Plant and Equipment [Line Items]
Generating capacity placed in service, in MWs | MWh				250
Plant in service		$ 444,000
RICE Units
Public Utility, Property, Plant and Equipment [Line Items]
Plant in service	$ 187,000		$ 187,000
Number of units in service | generating_units	10		10
Generating capacity (MW) | MWh	188		188