Employee Benefit Plans: (Tables)

9 Months Ended

Sep. 30, 2015

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 and nine months ended September 30, 2015 and 2014 are as follows:

	For The Three Months Ended September 30,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2015			2014			2015			2014			2015		2014
Components of Net Periodic Benefits Cost:
Service cost	$	1,569		$	1,411		$	65		$	63		$	204	$	192
Interest cost	1,922			1,880			99			111			163		154
Expected return on plan assets	(2,449		)	(2,225		)	(123		)	(114		)	—		—
Amortization of transition	—			—			—			105			—		—
Amortization of prior service cost (benefit)	30			30			(50		)	(50		)	29		40
Amortization of actuarial (gain) loss	448			—			(131		)	(115		)	108		35
Net periodic pension cost under accounting standards	1,520			1,096			(140		)	—			504		421
Regulatory adjustment — deferred	130			374			—			—			—		—
Total expense recognized, before allocation to overhead pool	$	1,650		$	1,470		$	(140	)	$	—		$	504	$	421

	For The Nine Months Ended September 30,
	Pension Benefits						Other Postretirement Benefits						SERP
(dollars in thousands)	2015			2014			2015			2014			2015		2014
Components of Net Periodic Benefits Cost:
Service cost	$	4,707		$	4,233		$	255		$	261		$	612	$	576
Interest cost	5,766			5,640			327			371			489		462
Expected return on plan assets	(7,347		)	(6,675		)	(369		)	(340		)	—		—
Amortization of transition	—			—			—			313			—		—
Amortization of prior service cost (benefit)	90			89			(150		)	(150		)	87		120
Amortization of actuarial (gain) loss	1,344			—			(237		)	(247		)	324		105
Net periodic pension cost under accounting standards	4,560			3,287			(174		)	208			1,512		1,263
Regulatory adjustment — deferred	392			1,123			—			—			—		—
Total expense recognized, before allocation to overhead pool	$	4,952		$	4,410		$	(174	)	$	208		$	1,512	$	1,263