Retirement Plans and Other Postretirement Benefits (Tables)

9 Months Ended

Sep. 30, 2016

Compensation and Retirement Disclosure [Abstract]

Schedule of net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset)

The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset) (dollars in thousands):

	Pension Benefits												Other Benefits
	Three Months Ended   September 30,						Nine Months Ended   September 30,						Three Months Ended   September 30,						Nine Months Ended   September 30,
	2016			2015			2016			2015			2016			2015			2016			2015
Service cost — benefits earned during the period	$	13,448		$	14,907		$	40,344		$	44,721		$	3,748		$	4,207		$	11,245		$	12,620
Interest cost on benefit obligation	32,912			30,996			98,735			92,987			7,430			7,026			22,291			21,077
Expected return on plan assets	(43,477		)	(44,808		)	(130,429		)	(134,424		)	(9,123		)	(9,214		)	(27,371		)	(27,641		)
Amortization of:
Prior service cost	132			148			395			446			(9,471		)	(9,492		)	(28,413		)	(28,476		)
Net actuarial loss	10,179			7,764			30,538			23,292			1,147			1,220			3,442			3,661
Net periodic benefit cost	$	13,194		$	9,007		$	39,583		$	27,022		$	(6,269	)	$	(6,253	)	$	(18,806	)	$	(18,759	)
Portion of cost charged to expense	$	6,476		$	4,433		$	19,427		$	15,653		$	(3,077	)	$	(3,078	)	$	(9,230	)	$	(7,348	)