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

12 Months Ended

Dec. 31, 2025

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 summarize the fair value of the nuclear decommissioning trusts and other special use fund assets (dollars in thousands): 

	December 31, 2025
	Fair Value							Total Unrealized Gains		Total Unrealized Losses
Investment Type:	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
Equity securities	$	519,562	$	62,573	$	582,135		$	433,044	$	(1)
Available for sale-fixed income securities	898,403		369,055		1,267,458		(a)	18,765		(14,993)
Other	(3,799)		3,199		(600)		(b)	—		—
Total	$	1,414,166	$	434,827	$	1,848,993	(c)	$	451,809	$	(14,994)

(a)As of December 31, 2025, the amortized cost basis of these available-for-sale investments is $1,265 million.

(b)Represents net pending securities sales and purchases.

(c)All amounts pertain to APS, with the exception of $40.3 million of other special use fund investments in equity securities and $3.2 million of unrealized gains relating to investments held by the Captive.

	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 $34.2 million of other special use fund investments in equity securities relating to investments held by the Captive.

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

The following table sets forth realized gains and losses relating to the sale and maturity of available-for-sale debt securities and equity securities, and the proceeds from the sale and maturity of these investment securities (dollars in thousands):

	Year Ended December 31,
	Nuclear Decommissioning Trusts		Other Special Use Funds			Total
2025
Realized gains	$	12,826	$	242		$	13,068
Realized losses	$	(11,749)	$	—		$	(11,749)
Proceeds from the sale of securities (a)	$	1,478,088	$	377,112	(b)	$	1,855,200
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

(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.

(b)All amounts pertain to APS, with the exception of $51.4 million of other special use fund proceeds from the sale of securities relating to investments held by Captive.

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

The fair value fixed income securities summarized by contractual maturities as of December 31, 2025 is as follows (dollars in thousands):

	Nuclear Decommissioning Trusts		Coal Reclamation Escrow Account		Active Union Employee Medical Account		Total
Less than one year	$	36,726	$	87,421	$	39,617	$	163,764
1 year – 5 years	272,413		68,348		157,116		497,877
5 years – 10 years	173,131		—		16,553		189,684
Greater than 10 years	416,133		—		—		416,133
Total	$	898,403	$	155,769	$	213,286	$	1,267,458