Employee Benefit Plans: (Tables)

3 Months Ended

Mar. 31, 2024

Retirement Benefits [Abstract]

Schedule of components of net periodic benefit costs, before allocation to the overhead pool, for Registrant's pension plan, postretirement plan, and SERP

The components of net periodic benefit costs for Registrant’s pension plan, postretirement medical benefit plan and SERP for the three months ended March 31, 2024 and 2023 were as follows:

	For The Three Months Ended March 31,
	Pension Benefits				Other Postretirement Benefits				SERP
(dollars in thousands)	2024		2023		2024		2023		2024		2023
Components of Net Periodic Benefits Cost:
Service cost	$	850	$	846	$	32	$	33	$	358	$	312
Interest cost	2,550		2,513		23		25		426		411
Expected return on plan assets	(3,009)		(2,623)		(142)		(120)		—		—
Amortization of prior service cost	108		108		—		—		—		—
Amortization of actuarial (gain) loss	—		—		(263)		(240)		(4)		(8)
Net periodic benefits costs under accounting standards	499		844		(350)		(302)		780		715
Regulatory adjustments - deferred	—		(92)		—		—		—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	499	$	752	$	(350)	$	(302)	$	780	$	715