ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended		6 Months Ended	9 Months Ended
	May 01, 2024	Sep. 30, 2024	Oct. 31, 2017	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 826,476		$ 826,476			$ 826,476		$ 919,483
Regulatory liabilities		584,375		584,375			584,375		2,317
Net regulatory asset		242,101		242,101			242,101		917,166
Goodwill impairment				68,223	$ 0		$ 68,223	$ 0
Amount received for contract termination payment	$ 709,400
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		1,507		1,507			$ 1,507		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	405,478		405,478			405,478		463
Transmission credit
Regulatory Asset and Liability [Line Items]
Regulatory liabilities		177,390		177,390			177,390		0
Deferred income tax expense
Regulatory Asset and Liability [Line Items]
Regulatory assets		14,609		14,609			14,609		15,223
Deferred prepaid lease expense | Springerville Unit 3 Lease
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 72,833		$ 72,833			72,833		74,551
Annual amortization expense of regulatory asset							$ 2,300
Amortization period		47 years		47 years			47 years
Acquisition costs | J.M. Shafer
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 0		$ 0			$ 0		37,749
Goodwill impairment		35,800
Acquisition costs | Colowyo Coal
Regulatory Asset and Liability [Line Items]
Regulatory assets		0		0			0		33,062
Goodwill impairment		32,400
Deferred debt prepayment transaction costs
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 99,946		$ 99,946			99,946		106,417
Annual amortization expense of regulatory asset							$ 8,600
Amortization period		21 years 4 months 24 days		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		$ 71,289		$ 71,289			71,289		74,795
Annual amortization expense of regulatory asset							$ 4,700
Amortization period		20 years		20 years			20 years
Deferred impairment loss | J.M. Shafer
Regulatory Asset and Liability [Line Items]
Amortization period		25 years		25 years			25 years
Deferred impairment loss | Unrecovered Plants
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 523,229		$ 523,229			$ 523,229		532,817
Environmental Obligation
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 44,570		$ 44,570			44,570		$ 44,869
Annual amortization expense of regulatory asset							$ 1,800
Amortization period		25 years		25 years			25 years