Employee Benefit Plans: (Tables)

6 Months Ended

Jun. 30, 2022

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 and six months ended June 30, 2022 and 2021 were as follows:

	For The Three Months Ended June 30,
	Pension Benefits				Other Postretirement Benefits				SERP
(dollars in thousands)	2022		2021		2022		2021		2022		2021
Components of Net Periodic Benefits Cost:
Service cost	$	1,342	$	1,625	$	33	$	40	$	298	$	348
Interest cost	1,856		1,712		16		31		256		229
Expected return on plan assets	(3,290)		(3,134)		(147)		(134)		—		—
Amortization of prior service cost	109		109		—		—		—		—
Amortization of actuarial (gain) loss	—		993		(412)		(287)		145		419
Net periodic benefits costs under accounting standards	17		1,305		(510)		(350)		699		996
Regulatory adjustment - deferred	—		(351)		—		—		—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	17	$	954	$	(510)	$	(350)	$	699	$	996
	For The Six Months Ended June 30,
	Pension Benefits				Other Postretirement Benefits				SERP
(dollars in thousands)	2022		2021		2022		2021		2022		2021
Components of Net Periodic Benefits Cost:
Service cost	$	2,822	$	3,250	$	66	$	80	$	596	$	696
Interest cost	3,700		3,424		32		62		512		458
Expected return on plan assets	(6,582)		(6,268)		(294)		(268)		—		—
Amortization of prior service cost	218		218		—		—		—		—
Amortization of actuarial (gain) loss	—		1,986		(824)		(574)		290		838
Net periodic benefits costs under accounting standards	158		2,610		(1,020)		(700)		1,398		1,992
Regulatory adjustment - deferred	—		(702)		—		—		—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	158	$	1,908	$	(1,020)	$	(700)	$	1,398	$	1,992