ACCOUNTING FOR RATE REGULATION (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 31, 2017	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 911,418		$ 919,483
Regulatory liabilities		2,199		2,317
Net regulatory asset		909,219		917,166
Rate stabilization
Regulatory Asset and Liability [Line Items]
Revenue from contract with customer		$ 0	$ 11,757
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		$ 1,736		1,854
Proceeds from settlement of interest rate swaps	$ 4,600
Deferred membership withdrawal income
Regulatory Asset and Liability [Line Items]
Regulatory liabilities		463		463
Deferred income tax expense
Regulatory Asset and Liability [Line Items]
Regulatory assets		15,223		15,223
Deferred prepaid lease expense | Springerville Unit 3 Lease
Regulatory Asset and Liability [Line Items]
Regulatory assets		73,979		74,551
Annual amortization expense of regulatory asset		$ 2,300
Amortization period		47 years
Acquisition costs | J.M. Shafer
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 37,037		37,749
Annual amortization expense of regulatory asset		$ 2,800
Amortization period		25 years
Acquisition costs | Colowyo Coal
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 32,803		33,062
Annual amortization expense of regulatory asset		$ 1,000
Amortization period		44 years
Deferred debt prepayment transaction costs
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 104,260		106,417
Annual amortization expense of regulatory asset		$ 8,600
Amortization period		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		73,626		74,795
Annual amortization expense of regulatory asset		$ 4,700
Amortization period		20 years
Deferred impairment loss | J.M. Shafer
Regulatory Asset and Liability [Line Items]
Amortization period		25 years
Deferred impairment loss | Unrecovered Plants
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 529,621		532,817
Environmental Obligation
Regulatory Asset and Liability [Line Items]
Regulatory assets		44,869		$ 44,869
Annual amortization expense of regulatory asset		$ 1,800
Amortization period		25 years