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Retirement Plans and Other Postretirement Benefits
 3 Months Ended
Mar. 31, 2024
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 other postretirement benefit plans for the employees of Pinnacle West and our subsidiaries.  The other postretirement benefit plans include a group life and medical plan and a post-65 retiree health reimbursement arrangement (“HRA”). Pinnacle West uses a December 31 measurement date each year for its pension and other postretirement benefit plans.  The market-related value of our plan assets is their fair value at the measurement date.
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 BenefitsOther Benefits
 Three Months Ended
March 31,		Three Months Ended
March 31,
 2024202320242023
Service cost — benefits earned during the period$10,631 $10,157 $2,459 $2,120 
Non-service costs (credits):
Interest cost on benefit obligation37,236 38,362 5,613 5,673 
Expected return on plan assets(46,983)(45,561)(11,709)(10,872)
  Amortization of:   
  Prior service credit (a)— — (9,447)(9,447)
  Net actuarial loss/(gain)10,944 9,713 (2,080)(2,303)
Net periodic cost/(benefit)$11,828 $12,671 $(15,164)$(14,829)
Portion of cost/(benefit) charged to expense$6,337 $7,227 $(11,306)$(10,737)
 
(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.

Contributions
 
We have not made any voluntary contributions to our pension plan year-to-date in 2024. The minimum required contributions for the pension plan are zero for the next three years and we do not expect to make any contributions in 2024, 2025 or 2026. With regard to contributions to our other postretirement benefit plan, we have not made a contribution year-to-date in 2024 and do not expect to make any contributions in 2024, 2025 or 2026.