Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

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.