v3.25.1
Retirement Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2025
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 or Charged or Amortized to the Regulatory Asset)
The following table provides detail 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 PlansOther Benefits Plans
Three Months Ended March 31,Three Months Ended March 31,
 2025202420252024
Service cost-benefits earned during the period$10,949 $10,631 $1,982 $2,459 
Non-service costs (credits):
Interest cost on benefit obligation38,976 37,236 5,102 5,613 
Expected return on plan assets(44,547)(46,983)(12,142)(11,709)
Amortization of:
Prior service credit (a)— — (1,265)(9,447)
Net actuarial loss (gain)
12,118 10,944 (2,899)(2,080)
Net periodic benefit costs (credits)
$17,496 $11,828 $(9,222)$(15,164)
Portion of costs (credits) charged to expense
$10,461 $6,337 $(6,873)$(11,306)
(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 and they became fully amortized as of January 31, 2025.