SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 919,483	$ 650,421
Regulatory liabilities		2,317	49,931
Net regulatory asset		917,166	600,490
Impairment losses of long-lived assets		261,600	29,098	$ 0
Rate stabilization
Regulatory Asset and Liability [Line Items]
Revenue from contract with customer		$ 47,127	95,613	$ 78,457
First Mortgage Obligations, Series 2017A, Tranche 1, 3.34%, due through 2029 | Secured Debt
Regulatory Asset and Liability [Line Items]
Term of issuance		12 years
Rifle Generating Station
Regulatory Asset and Liability [Line Items]
Impairment losses of long-lived assets			3,700
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		1,854	2,341
Deferral of realized gain	$ 4,600
Membership Withdrawal
Regulatory Asset and Liability [Line Items]
Regulatory liabilities		463	47,590
Deferred Revenue | Rate stabilization
Regulatory Asset and Liability [Line Items]
Revenue from contract with customer		47,100
Deferred Income Tax Expense
Regulatory Asset and Liability [Line Items]
Regulatory assets		15,223	19,279
Deferred Prepaid Lease Expense | Springerville Unit 3 Lease
Regulatory Asset and Liability [Line Items]
Regulatory assets		74,551	76,842
Annual amortization expense		$ 2,300
Amortization period		47 years
Goodwill | Thermo Cogeneration Partnership
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 37,749	40,598
Annual amortization expense		$ 2,800
Amortization period		25 years
Goodwill | Colowyo Coal
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 33,062	34,095
Annual amortization expense		$ 1,000
Amortization period		44 years
Deferred Debt Prepayment Transaction Costs
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 106,417	115,045
Annual amortization expense		$ 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		$ 12,200
Amortization period		25 years
Deferred Impairment Loss | Rifle Generating Station
Regulatory Asset and Liability [Line Items]
Annual amortization expense		$ 600
Deferred Impairment Loss | Nucla and Escalante Generating Stations
Regulatory Asset and Liability [Line Items]
Regulatory assets		532,817	285,092
Deferred Impairment Loss | Holcomb Expansion
Regulatory Asset and Liability [Line Items]
Regulatory assets		74,795	79,470
Annual amortization expense		$ 4,700
Amortization period		20 years
Environmental Obligation
Regulatory Asset and Liability [Line Items]
Regulatory assets		$ 44,869	$ 0
Annual amortization expense		$ 1,800
Amortization period		25 years