Employee Benefit Plans: (Tables)

9 Months Ended

Sep. 30, 2019

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 plan and SERP for the three and nine months ended September 30, 2019 and 2018 were as follows:

	For The Three Months Ended September 30,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2019			2018			2019			2018			2019		2018
Components of Net Periodic Benefits Cost:
Service cost	$	1,110		$	1,335		$	53		$	48		$	298	$	274
Interest cost	2,132			1,912			80			75			267		222
Expected return on plan assets	(2,594		)	(2,793		)	(112		)	(123		)	—		—
Amortization of prior service cost	109			—			—			—			—		—
Amortization of actuarial (gain) loss	355			314			(150		)	(212		)	118		262
Net periodic benefits costs under accounting standards	1,112			768			(129		)	(212		)	683		758
Regulatory adjustment - deferred	(160		)	—			—			—			—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	952		$	768		$	(129	)	$	(212	)	$	683	$	758

	For The Nine Months Ended September 30,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2019			2018			2019			2018			2019		2018
Components of Net Periodic Benefits Cost:
Service cost	$	3,330		$	4,005		$	159		$	162		$	894	$	822
Interest cost	6,396			5,736			240			219			801		666
Expected return on plan assets	(7,782		)	(8,379		)	(336		)	(369		)	—		—
Amortization of prior service cost	327			—			—			—			—		—
Amortization of actuarial (gain) loss	1,065			942			(450		)	(576		)	354		786
Net periodic benefits costs under accounting standards	3,336			2,304			(387		)	(564		)	2,049		2,274
Regulatory adjustment - deferred	(502		)	—			—			—			—		—
Total expense (benefit) recognized, before surcharges and allocation to overhead pool	$	2,834		$	2,304		$	(387	)	$	(564	)	$	2,049	$	2,274