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Retirement Plans and Other Postretirement Benefits (Tables) | 6 Months Ended
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Jun. 30, 2024
Retirement Benefits [Abstract] |
Schedule of Net Periodic Benefit Costs and The Portion of These Costs Charged To Expense (Including Administrative Costs and Excluding Amounts Capitalized as Overhead Construction, Billed To Electric Plant Participants) | The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):
| Pension Benefits |  | Other Benefits
| Three Months Ended June 30, |  | Six Months Ended June 30, |  | Three Months Ended June 30, |  | Six Months Ended June 30,
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Service cost — benefits earned during the period | $ | 11,190 |  |  | $ | 9,573 |  |  | $ | 21,821 |  |  | $ | 19,730 |  |  | $ | 2,518 |  |  | $ | 2,164 |  |  | $ | 4,977 |  |  | $ | 4,284 |
Non-service costs (credits): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest cost on benefit obligation | 37,085 |  |  | 38,418 |  |  | 74,321 |  |  | 76,780 |  |  | 5,472 |  |  | 5,582 |  |  | 11,085 |  |  | 11,255 |
Expected return on plan assets | (47,342) |  |  | (45,908) |  |  | (94,325) |  |  | (91,469) |  |  | (11,708) |  |  | (10,872) |  |  | (23,417) |  |  | (21,744) |
Amortization of: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prior service credit (a) | — |  |  | — |  |  | — |  |  | — |  |  | (9,447) |  |  | (9,447) |  |  | (18,894) |  |  | (18,894) |
Net actuarial loss/(gain) | 10,014 |  |  | 9,497 |  |  | 20,958 |  |  | 19,210 |  |  | (2,258) |  |  | (2,504) |  |  | (4,338) |  |  | (4,807) |
Net periodic cost/(benefit) | $ | 10,947 |  |  | $ | 11,580 |  |  | $ | 22,775 |  |  | $ | 24,251 |  |  | $ | (15,423) |  |  | $ | (15,077) |  |  | $ | (30,587) |  |  | $ | (29,906) |
Portion of cost/(benefit) charged to expense | $ | 5,500 |  |  | $ | 6,513 |  |  | $ | 11,837 |  |  | $ | 13,740 |  |  | $ | (11,515) |  |  | $ | (10,948) |  |  | $ | (22,821) |  |  | $ | (21,685) |
(a)Prior-service costs or credits reflect the impact of modifications to the pension or postretirement plan benefits. The impact of these modifications is amortized over a period which reflects the demographics of the impacted population. In 2014, Pinnacle West made changes to the postretirement benefits offered to Medicare eligible retirees, which resulted in prior-service credits. We have been amortizing these prior-serviced credits since 2015 with the last full-year amortization occurring in 2024.

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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