Employee Benefit Plans: (Tables)

3 Months Ended

Mar. 31, 2023

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, 2023 and 2022 were as follows:

	For The Three Months Ended March 31,
	Pension Benefits				Other Postretirement Benefits				SERP
(dollars in thousands)	2023		2022		2023		2022		2023		2022
Components of Net Periodic Benefits Cost:
Service cost	$	846	$	1,480	$	33	$	33	$	312	$	298
Interest cost	2,513		1,844		25		16		411		256
Expected return on plan assets	(2,623)		(3,292)		(120)		(147)		—		—
Amortization of prior service cost	108		109		—		—		—		—
Amortization of actuarial (gain) loss	—		—		(240)		(412)		(8)		145
Net periodic benefits costs under accounting standards	844		141		(302)		(510)		715		699
Regulatory adjustment - deferred cost	(92)		—		—		—		—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	752	$	141	$	(302)	$	(510)	$	715	$	699