Employee Benefit Plans: (Tables)

3 Months Ended

Mar. 31, 2016

Compensation and Retirement Disclosure [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 Supplemental Executive Retirement Plan ("SERP") for the three months ended March 31, 2016 and 2015 are as follows:

	For The Three Months Ended March 31,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2016			2015			2016			2015			2016		2015
Components of Net Periodic Benefits Cost:
Service cost	$	1,232		$	1,686		$	68		$	95		$	200	$	204
Interest cost	1,930			1,939			97			114			186		163
Expected return on plan assets	(2,460		)	(2,446		)	(122		)	(123		)	—		—
Amortization of transition	—			—			—			—			—		—
Amortization of prior service cost (benefit)	12			30			(9		)	(50		)	6		29
Amortization of actuarial (gain) loss	127			469			(150		)	(53		)	73		108
Net periodic pension cost under accounting standards	841			1,678			(116		)	(17		)	465		504
Regulatory adjustment — deferred	359			11			—			—			—		—
Total expense recognized, before allocation to overhead pool	$	1,200		$	1,689		$	(116	)	$	(17	)	$	465	$	504