Retirement Plans and Other Postretirement Benefits (Tables)

3 Months Ended

Mar. 31, 2017

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 to the regulatory asset or liability) (dollars in thousands):

	Pension Benefits						Other Benefits
	Three Months Ended   March 31,						Three Months Ended   March 31,
	2017			2016			2017			2016
Service cost — benefits earned during the period	$	13,760		$	14,266		$	4,358		$	3,937
Interest cost on benefit obligation	32,701			32,945			7,565			7,341
Expected return on plan assets	(43,710		)	(43,792		)	(13,350		)	(9,122		)
Amortization of:
Prior service cost (credit)	20			132			(9,461		)	(9,471		)
Net actuarial loss	12,489			9,731			1,454			946
Net periodic benefit cost	$	15,260		$	13,282		$	(9,434	)	$	(6,369	)
Portion of cost charged to expense	$	7,568		$	6,519		$	(4,678	)	$	(3,126	)