Employee Benefit Plans: (Tables)

3 Months Ended

Mar. 31, 2018

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 months ended March 31, 2018 and 2017 are as follows:

	For The Three Months Ended March 31,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2018			2017			2018			2017			2018		2017
Components of Net Periodic Benefits Cost:
Service cost	$	1,401		$	1,251		$	57		$	59		$	274	$	232
Interest cost	1,921			1,967			72			85			222		223
Expected return on plan assets	(2,791		)	(2,610		)	(123		)	(122		)	—		—
Amortization of prior service cost (benefit)	—			—			—			—			—		3
Amortization of actuarial (gain) loss	345			209			(182		)	(170		)	262		194
Net periodic pension cost under accounting standards	876			817			(176		)	(148		)	758		652
Regulatory adjustment — deferred	—			433			—			—			—		—
Total expense recognized, before surcharges and allocation to overhead pool	$	876		$	1,250		$	(176	)	$	(148	)	$	758	$	652