Employee Benefit Plans: (Tables)

3 Months Ended

Mar. 31, 2020

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, 2020 and 2019 were as follows:

	For The Three Months Ended March 31,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2020			2019			2020			2019			2020		2019
Components of Net Periodic Benefits Cost:
Service cost	$	1,408		$	1,234		$	47		$	53		$	244	$	298
Interest cost	1,954			2,131			56			80			247		267
Expected return on plan assets	(2,950		)	(2,593		)	(127		)	(112		)	—		—
Amortization of prior service cost	109			109			—			—			—		—
Amortization of actuarial (gain) loss	442			359			(199		)	(150		)	211		118
Net periodic benefits costs under accounting standards	963			1,240			(223		)	(129		)	702		683
Regulatory adjustment - deferred	(93		)	—			—			—			—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	870		$	1,240		$	(223	)	$	(129	)	$	702	$	683