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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Tables)
 3 Months Ended
Mar. 31, 2026
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): 
March 31, 2026
 Fair ValueTotal
Unrealized
Gains		Total
Unrealized
Losses
Investment Type:Nuclear Decommissioning TrustsOther Special Use FundsTotal
Equity securities$497,311 $64,850 $562,161 $407,542 (d)$(139)
Available for sale-fixed income securities891,949 370,883 1,262,832 (a)11,497 (18,344)
Other3,557 2,012 5,569 (b)— — 
Total$1,392,817 $437,745 $1,830,562 (c)$419,039 $(18,483)
(a)As of March 31, 2026, the amortized cost basis of these available-for-sale investments is $1,270 million.
(b)Represents net pending securities sales and purchases.
(c)All amounts pertain to APS, with the exception of $41.4 million of other special use fund investments in equity securities relating to investments held by the Captive.
(d)All amounts pertain to APS, with the exception of $2.8 million of unrealized gains relating to investments held by the Captive.
December 31, 2025
 Fair ValueTotal
Unrealized
Gains		Total
Unrealized
Losses
Investment Type:Nuclear Decommissioning TrustsOther Special Use FundsTotal
Equity securities$519,562 $62,573 $582,135 $433,044 (d)$(1)
Available for sale-fixed income securities898,403 369,055 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 relating to investments held by the Captive.
(d)All amounts pertain to APS, with the exception of $3.2 million of unrealized gains 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):
 Three Months Ended March 31,
 Nuclear Decommissioning TrustsOther Special Use FundsTotal
2026
Realized gains$4,720 $16 $4,736 
Realized losses$(2,797)$— $(2,797)
Proceeds from the sale of securities (a)$447,883 $64,351 (b)$512,234 
2025
Realized gains$1,658 $— $1,658 
Realized losses$(2,761)$— $(2,761)
Proceeds from the sale of securities (a)$416,601 $69,213 (c)$485,814 
(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.
(c)    All amounts pertain to APS, with the exception of $25.3 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 March 31, 2026 is as follows (dollars in thousands):
 
 Nuclear Decommissioning TrustsCoal Reclamation Escrow AccountActive Union Employee Medical AccountTotal
Less than one year$32,191 $108,470 $39,753 $180,414 
1 year – 5 years299,067 49,806 156,363 505,236 
5 years – 10 years168,027 — 16,491 184,518 
Greater than 10 years392,664 — — 392,664 
Total$891,949 $158,276 $212,607 $1,262,832