ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended			6 Months Ended
	May 01, 2024	Oct. 31, 2017	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 903,354			$ 903,354		$ 919,483
Regulatory liabilities			693,835			693,835		2,317
Net regulatory asset			209,519			209,519		917,166
Amount received for contract termination payment	$ 709,400
Transmission credit portion of contract termination payment	179,300
Rate stabilization
Regulatory Asset and Liability [Line Items]
Revenue from contract with customer			16,901		$ 11,064	$ 16,901	$ 22,821
First Mortgage Obligations, Series 2017A , Tranche 1, 3.34%, due through 2029
Regulatory Asset and Liability [Line Items]
Term of issuance						12 years
Craig Generating Station Unit 2 And 3
Regulatory Asset and Liability [Line Items]
Impairment losses of long-lived assets				$ 261,600
Interest rate swaps, realized gain
Regulatory Asset and Liability [Line Items]
Regulatory liabilities			1,625			$ 1,625		1,854
Proceeds from settlement of interest rate swaps		$ 4,600
Deferred membership withdrawal income
Regulatory Asset and Liability [Line Items]
Regulatory liabilities	$ 530,100		513,699			513,699		463
Transmission credit
Regulatory Asset and Liability [Line Items]
Regulatory liabilities			178,511			178,511		0
Deferred income tax expense
Regulatory Asset and Liability [Line Items]
Regulatory assets			15,223			15,223		15,223
Deferred prepaid lease expense | Springerville Unit 3 Lease
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 73,406			73,406		74,551
Annual amortization expense of regulatory asset						$ 2,300
Amortization period			47 years			47 years
Acquisition costs | J.M. Shafer
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 36,325			$ 36,325		37,749
Annual amortization expense of regulatory asset						$ 2,800
Amortization period			25 years			25 years
Acquisition costs | Colowyo Coal
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 32,545			$ 32,545		33,062
Annual amortization expense of regulatory asset						$ 1,000
Amortization period			44 years			44 years
Deferred debt prepayment transaction costs
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 102,103			$ 102,103		106,417
Annual amortization expense of regulatory asset						$ 8,600
Amortization period			21 years 4 months 24 days			21 years 4 months 24 days
Deferred impairment loss | Escalante Generating Station
Regulatory Asset and Liability [Line Items]
Annual amortization expense of regulatory asset						$ 12,200
Deferred impairment loss | Rifle Generating Station
Regulatory Asset and Liability [Line Items]
Annual amortization expense of regulatory asset						600
Deferred impairment loss | Holcomb Expansion
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 72,458			72,458		74,795
Annual amortization expense of regulatory asset						$ 4,700
Amortization period			20 years			20 years
Deferred impairment loss | J.M. Shafer
Regulatory Asset and Liability [Line Items]
Amortization period			25 years			25 years
Deferred impairment loss | Unrecovered Plants
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 526,425			$ 526,425		532,817
Environmental Obligation
Regulatory Asset and Liability [Line Items]
Regulatory assets			$ 44,869			44,869		$ 44,869
Annual amortization expense of regulatory asset						$ 1,800
Amortization period			25 years			25 years