Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Tables)

12 Months Ended

Dec. 31, 2024

Investments, Debt and Equity Securities [Abstract]

Schedule of Fair Value of APS's Nuclear Decommissioning Trust Fund Assets

The following tables present the unrealized gains and losses based on the original cost of the investment and summarizes the fair value of the nuclear decommissioning trusts and other special use fund assets (dollars in thousands): 

	December 31, 2024
	Fair Value							Total Unrealized Gains		Total Unrealized Losses
Investment Type:	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
Equity securities	$	435,470	$	24,962	$	460,432		$	359,127	$	(176)
Available for sale-fixed income securities	844,040		355,544		1,199,584		(a)	7,717		(31,960)
Other	3,335		27,851		31,186		(b)	—		—
Total	$	1,282,845	$	408,357	$	1,691,202	(c)	$	366,844	$	(32,136)

(a)As of December 31, 2024, the amortized cost basis of these available-for-sale investments is $1,224 million.

(b)Represents net pending securities sales and purchases.

(c)All amounts pertain to APS, with the exception of $34 million of Other Special Use Fund investments in equity securities relating to the Captive.

	December 31, 2023
	Fair Value							Total Unrealized Gains		Total Unrealized Losses
Investment Type:	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
Equity securities	$	420,680	$	40,991	$	461,671		$	336,555	$	—
Available for sale-fixed income securities	781,333		319,594		1,100,927		(a)	21,518		(40,868)
Other	(767)		2,196		1,429		(b)	39		—
Total	$	1,201,246	$	362,781	$	1,564,027	(c)	$	358,112	$	(40,868)

(a)As of December 31, 2023, the amortized cost basis of these available-for-sale investments is $1,120 million.

(b)Represents net pending securities sales and purchases.

(c)All amounts pertain to APS.

Schedule of Realized Gains and Losses and Proceeds from the Sale of Securities by the Nuclear Decommissioning Trust Funds

	Year Ended December 31,
	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
2024
Realized gains	$	75,690	$	372	$	76,062
Realized losses	$	(21,966)	$	—	$	(21,966)
Proceeds from the sale of securities (a)	$	1,330,940	$	355,154	$	1,686,094
2023
Realized gains	$	111,922	$	172	$	112,094
Realized losses	$	(41,212)	$	(568)	$	(41,780)
Proceeds from the sale of securities (a)	$	1,324,978	$	354,744	$	1,679,722
2022
Realized gains	$	9,017	$	420	$	9,437
Realized losses	$	(40,239)	$	—	$	(40,239)
Proceeds from the sale of securities (a)	$	979,639	$	227,558	$	1,207,197

(a)Proceeds are reinvested in the nuclear decommissioning trusts and other special use funds, excluding amounts reimbursed to the Company for active union employee medical claims from the active union employee medical account.

Schedule of Fair Value of Fixed Income Securities, Summarized by Contractual Maturities

The fair value of APS’s fixed income securities, summarized by contractual maturities, at December 31, 2024, is as follows (dollars in thousands):

	Nuclear Decommissioning Trusts		Coal Reclamation Escrow Account		Active Union Employee Medical Account		Total
Less than one year	$	19,868	$	87,424	$	39,090	$	146,382
1 year – 5 years	268,974		58,598		154,768		482,340
5 years – 10 years	198,464		—		15,664		214,128
Greater than 10 years	356,734		—		—		356,734
Total	$	844,040	$	146,022	$	209,522	$	1,199,584