UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Utility Plant in Service by Major Class) (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) MWh generating_units	Dec. 31, 2019 USD ($)	May 31, 2021 MWh
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate		3.30%	3.15%	3.08%
Plant in Service		$ 7,797,935	$ 7,073,292
Plant acquisition adjustments		(206,000)	(202,000)
Amount reclassified	$ 33,000
Computer software, accumulated amortization		169,000	199,000
Amortization of computer software costs		33,000	$ 29,000	$ 26,000
Springerville Common Facilities
Public Utility, Property, Plant and Equipment [Line Items]
Percentage of interest purchased			32.20%
RICE Units
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service			$ 187,000
Number of units in service | generating_units			10
Generating capacity (MW) | MWh			188
Oso Grande
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service		$ 444,000
Generating capacity placed in service, in MWs | MWh					250
Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Renewable nominal generation capacity, including PPAs (MWs) | MWh					700
Smaller Application Software | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Amortization period		3 years
Smaller Application Software | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Amortization period		5 years
Large Enterprise Software
Public Utility, Property, Plant and Equipment [Line Items]
Amortization period		10 years
Generation Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate		3.11%
Average Remaining Life in Years		18 years
Plant in Service		$ 3,753,000	$ 3,279,000
Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate		1.93%
Average Remaining Life in Years		33 years
Plant in Service		$ 2,024,000	1,906,000
Transmission Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate		1.69%
Average Remaining Life in Years		35 years
Plant in Service		$ 1,210,000	1,090,000
General Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual Depreciation Rate		6.01%
Average Remaining Life in Years		7 years
Plant in Service		$ 540,000	503,000
Intangible Plant, Software Costs and Other
Public Utility, Property, Plant and Equipment [Line Items]
Average Remaining Life in Years		3 years
Plant in Service		$ 268,000	291,000
Plant acquisition adjustments		(4,000)	0
Future estimated amortization costs for intangible assets
2022		26,000
2023		19,000
2024		12,000
2025		7,000
2026		4,000
Plant Held for Future Use
Public Utility, Property, Plant and Equipment [Line Items]
Plant in Service		$ 3,000	$ 4,000