Employee Benefit Plans: (Tables)

6 Months Ended

Jun. 30, 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, before allocation to the overhead pool, for Registrant’s pension plan, postretirement plan and the supplemental executive retirement plan ("SERP") for the three and six months ended June 30, 2018 and 2017 are as follows:

	For The Three Months Ended June 30,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2018			2017			2018			2017			2018		2017
Components of Net Periodic Benefits Cost:
Service cost	$	1,269		$	1,249		$	57		$	59		$	274	$	232
Interest cost	1,903			1,985			72			85			222		223
Expected return on plan assets	(2,795		)	(2,240		)	(123		)	(122		)	—		—
Amortization of prior service cost (benefit)	—			—			—			—			—		3
Amortization of actuarial (gain) loss	283			253			(182		)	(170		)	262		194
Net periodic pension cost under accounting standards	660			1,247			(176		)	(148		)	758		652
Regulatory adjustment — deferred	—			92			—			—			—		—
Total expense recognized, before surcharges and allocation to overhead pool	$	660		$	1,339		$	(176	)	$	(148	)	$	758	$	652

	For The Six Months Ended June 30,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2018			2017			2018			2017			2018		2017
Components of Net Periodic Benefits Cost:
Service cost	$	2,670		$	2,500		$	114		$	118		$	548	$	464
Interest cost	3,824			3,952			144			170			444		446
Expected return on plan assets	(5,586		)	(4,850		)	(246		)	(244		)	—		—
Amortization of prior service cost (benefit)	—			—			—			—			—		6
Amortization of actuarial (gain) loss	628			462			(364		)	(340		)	524		388
Net periodic pension cost under accounting standards	1,536			2,064			(352		)	(296		)	1,516		1,304
Regulatory adjustment — deferred	—			525			—			—			—		—
Total expense recognized, before surcharges and allocation to overhead pool	$	1,536		$	2,589		$	(352	)	$	(296	)	$	1,516	$	1,304