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

6 Months Ended

Jun. 30, 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): 

	June 30, 2025
	Fair Value							Total Unrealized Gains		Total Unrealized Losses
Investment Type:	Nuclear Decommissioning Trusts		Other Special Use Funds		Total
Equity securities	$	467,967	$	64,322	$	532,289		$	385,464	$	—
Available for sale-fixed income securities	866,215		356,166		1,222,381		(a)	15,242		(19,502)
Other	3,676		2,844		6,520		(b)	—		—
Total	$	1,337,858	$	423,332	$	1,761,190	(c)	$	400,706	$	(19,502)

(a)As of June 30, 2025, the amortized cost basis of these available-for-sale investments is $1,227 million.

(b)Represents net pending securities sales and purchases.

(c)All amounts pertain to APS, with the exception of $38.5 million of other special use fund investments in equity securities and $2.0 million of unrealized gains relating to 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 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):

	Three Months Ended June 30,
	Nuclear Decommissioning Trusts		Other Special Use Funds			Total
2025
Realized gains	$	1,702	$	—		$	1,702
Realized losses	$	(2,611)	$	—		$	(2,611)
Proceeds from the sale of securities (a)	$	342,313	$	91,517	(b)	$	433,830
2024
Realized gains	$	8,943	$	—		$	8,943
Realized losses	$	(3,706)	$	—		$	(3,706)
Proceeds from the sale of securities (a)	$	270,631	$	57,874		$	328,505

(a)    Proceeds are reinvested in the nuclear decommissioning trusts and other special use funds, excluding investment fees and 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 $25.2 million of other special use fund proceeds from the sale of securities relating to the Captive.

	Six Months Ended June 30,
	Nuclear Decommissioning Trusts		Other Special Use Funds			Total
2025
Realized gains	$	3,360	$	—		$	3,360
Realized losses	$	(5,372)	$	—		$	(5,372)
Proceeds from the sale of securities (a)	$	758,914	$	160,730	(b)	$	919,644
2024
Realized gains	$	63,435	$	80		$	63,515
Realized losses	$	(6,521)	$	—		$	(6,521)
Proceeds from the sale of securities (a)	$	648,453	$	123,922		$	772,375

(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 $50.5 million of other special use fund proceeds from the sale of securities relating to the 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 June 30, 2025 is as follows (dollars in thousands):

	Nuclear Decommissioning Trusts		Coal Reclamation Escrow Account		Active Union Employee Medical Account		Total
Less than one year	$	19,613	$	83,392	$	39,865	$	142,870
1 year – 5 years	278,017		58,611		158,159		494,787
5 years – 10 years	171,593		—		16,139		187,732
Greater than 10 years	396,992		—		—		396,992
Total	$	866,215	$	142,003	$	214,163	$	1,222,381