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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds - Schedule of Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Variable Interest Entity | Pinnacle West Captive Insurance Cell |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Special use fund | $ 39,000 |  | $ 39,000 |  | $ 34,000
APS |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Total | 1,823,378 |  | 1,823,378 |  | 1,691,202
Total Unrealized Gains |  |  | 440,645 |  | 366,844
Total Unrealized Losses |  |  | (14,982) |  | (32,136)
Amortized cost | 1,205,000 |  | 1,205,000 |  | 1,224,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 3,972 | $ 3,825 | 7,332 | $ 67,340 |
Realized losses | (2,184) | (4,162) | (7,556) | (10,683) |
Proceeds from the sale of securities | 455,515 | 367,996 | 1,375,159 | 1,140,371 |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Special use fund |  |  |  |  | 34,200
APS | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Total | 1,393,446 |  | 1,393,446 |  | 1,282,845
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 3,730 | 3,825 | 7,090 | 67,260 |
Realized losses | (2,184) | (4,162) | (7,556) | (10,683) |
Proceeds from the sale of securities | 357,399 | 293,758 | 1,116,313 | 942,211 |
APS | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Total | 429,932 |  | 429,932 |  | 408,357
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 242 | 0 | 242 | 80 |
Realized losses | 0 | 0 | 0 | 0 |
Proceeds from the sale of securities | 98,116 | $ 74,238 | 258,846 | $ 198,160 |
APS | Captive Insurance Cell |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Total Unrealized Gains |  |  | 2,500 |  |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Proceeds from the sale of securities |  |  | 50,500 |  |
APS | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 610,185 |  | 610,185 |  | 460,432
Total Unrealized Gains |  |  | 421,222 |  | 359,127
Total Unrealized Losses |  |  | 0 |  | (176)
APS | Equity securities | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 506,831 |  | 506,831 |  | 435,470
APS | Equity securities | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 103,354 |  | 103,354 |  | 24,962
APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Available for sale-fixed income securities | 1,208,935 |  | 1,208,935 |  | 1,199,584
Total Unrealized Gains |  |  | 19,423 |  | 7,717
Total Unrealized Losses |  |  | (14,982) |  | (31,960)
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 174,969 |  | 174,969 |  |
1 year – 5 years | 455,343 |  | 455,343 |  |
5 years – 10 years | 166,081 |  | 166,081 |  |
Greater than 10 years | 412,542 |  | 412,542 |  |
Total | 1,208,935 |  | 1,208,935 |  |
APS | Available for sale-fixed income securities | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Available for sale-fixed income securities | 884,524 |  | 884,524 |  | 844,040
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 34,450 |  | 34,450 |  |
1 year – 5 years | 271,451 |  | 271,451 |  |
5 years – 10 years | 166,081 |  | 166,081 |  |
Greater than 10 years | 412,542 |  | 412,542 |  |
Total | 884,524 |  | 884,524 |  |
APS | Available for sale-fixed income securities | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Available for sale-fixed income securities | 324,411 |  | 324,411 |  | 355,544
APS | Available for sale-fixed income securities | Coal Reclamation Escrow Account |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 101,134 |  | 101,134 |  |
1 year – 5 years | 48,192 |  | 48,192 |  |
5 years – 10 years | 0 |  | 0 |  |
Greater than 10 years | 0 |  | 0 |  |
Total | 149,326 |  | 149,326 |  |
APS | Available for sale-fixed income securities | Active Union Employee Medical Account |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 39,385 |  | 39,385 |  |
1 year – 5 years | 135,700 |  | 135,700 |  |
5 years – 10 years | 0 |  | 0 |  |
Greater than 10 years | 0 |  | 0 |  |
Total | 175,085 |  | 175,085 |  |
APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Other | 4,258 |  | 4,258 |  | 31,186
Total Unrealized Gains |  |  | 0 |  | 0
Total Unrealized Losses |  |  | 0 |  | 0
APS | Other | Nuclear Decommissioning Trusts |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Other | 2,091 |  | 2,091 |  | 3,335
APS | Other | Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Other | 2,167 |  | 2,167 |  | $ 27,851
Pinnacle West | Variable Interest Entity | Pinnacle West Captive Insurance Cell |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Special use fund | $ 39,400 |  | $ 39,400 |  |

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