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Retirement Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2025
Retirement Benefits [Abstract]  
Schedule of Net Periodic Benefit Costs and the Portion of these Costs Charged to Expense
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 September 30,Nine Months Ended September 30,Three Months Ended September 30,Nine Months Ended September 30,
 20252024202520242025202420252024
Service cost-benefits earned during the period$11,038 $10,910 $33,114 $32,731 $2,020 $2,489 $6,061 $7,466 
Non-service costs (credits):
Interest cost on benefit obligation38,780 37,161 116,340 111,482 5,086 5,542 15,258 16,627 
Expected return on plan assets(44,698)(47,163)(134,094)(141,488)(12,142)(11,709)(36,426)(35,126)
Amortization of:
Prior service credit (a)— — — — — (9,447)(1,265)(28,341)
Net actuarial loss (gain)
11,683 10,479 35,049 31,437 (2,932)(2,169)(8,796)(6,507)
Net periodic benefit costs (credits)
$16,803 $11,387 $50,409 $34,162 $(7,968)$(15,294)$(25,168)$(45,881)
Portion of costs (credits) charged to expense
$10,270 $5,828 $30,096 $17,665 $(6,110)$(11,396)$(19,382)$(34,217)
(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.