Retirement Plans and Other Postretirement Benefits (Tables)

3 Months Ended

Mar. 31, 2026

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 Plans				Other Benefits Plans
	Three Months Ended March 31,				Three Months Ended March 31,
	2026		2025		2026		2025
Service cost-benefits earned during the period	$	12,099	$	10,949	$	2,237	$	1,982
Non-service costs (credits):
Interest cost on benefit obligation	37,612		38,976		4,979		5,102
Expected return on plan assets	(44,096)		(44,547)		(12,719)		(12,142)
Amortization of:
Prior service cost (credit)	151		—		—		(1,265)
Net actuarial loss (gain)	11,153		12,118		(2,700)		(2,899)
Net periodic benefit costs (credits)	$	16,919	$	17,496	$	(8,203)	$	(9,222)
Portion of costs (credits) charged to expense	$	9,518	$	10,461	$	(6,509)	$	(6,873)