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Retirement Plans and Other Postretirement Benefits | 9 Months Ended
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Sep. 30, 2020
Retirement Benefits [Abstract] |
Retirement Plans and Other Postretirement Benefits | Retirement Plans and Other Postretirement Benefits Pinnacle West sponsors a qualified defined benefit and account balance pension plan, a non-qualified supplemental excess benefit retirement plan, and an other postretirement benefit plan for the employees of Pinnacle West and our subsidiaries. Pinnacle West uses a December 31 measurement date for its pension and other postretirement benefit plans. The market-related value of our plan assets is their fair value at the measurement dates. 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 September 30, |  | Nine Months Ended September 30, |  | Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019 |  | 2020 |  | 2019
Service cost — benefits earned during the period | $ | 14,058 |  |  | $ | 12,476 |  |  | $ | 42,174 |  |  | $ | 37,427 |  |  | $ | 5,559 |  |  | $ | 4,593 |  |  | $ | 16,677 |  |  | $ | 13,777 |
Non-service costs (credits): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest cost on benefit obligation | 29,642 |  |  | 34,211 |  |  | 88,925 |  |  | 102,632 |  |  | 6,464 |  |  | 7,473 |  |  | 19,393 |  |  | 22,420 |
Expected return on plan assets | (46,861) |  |  | (42,971) |  |  | (140,582) |  |  | (128,913) |  |  | (10,019) |  |  | (9,603) |  |  | (30,057) |  |  | (28,809) |
Amortization of: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prior service credit | — |  |  | — |  |  | — |  |  | — |  |  | (9,394) |  |  | (9,456) |  |  | (28,182) |  |  | (28,366) |
Net actuarial loss | 8,653 |  |  | 10,646 |  |  | 25,959 |  |  | 31,938 |  |  | — |  |  | — |  |  | — |  |  | — |
Net periodic benefit cost (credit) | $ | 5,492 |  |  | $ | 14,362 |  |  | $ | 16,476 |  |  | $ | 43,084 |  |  | $ | (7,390) |  |  | $ | (6,993) |  |  | $ | (22,169) |  |  | $ | (20,978) |
Portion of cost (credit) charged to expense | $ | 736 |  |  | $ | 7,593 |  |  | $ | 2,349 |  |  | $ | 22,837 |  |  | $ | (5,286) |  |  | $ | (4,966) |  |  | $ | (15,798) |  |  | $ | (14,846) |
Contributions We have made voluntary contributions of $100 million to our pension plan year-to-date in 2020. The minimum required contributions for the pension plan are zero for the 2020-2022 period. We expect to make voluntary contributions up to $100 million per year during this period. We do not expect to make any contributions over this period to our other postretirement benefit plans.

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