Retirement Plans and Other Postretirement Benefits (Tables)

3 Months Ended

Mar. 31, 2018

Retirement Benefits [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 or billed to electric plant participants) (dollars in thousands):

	Pension Benefits						Other Benefits
	Three Months Ended   March 31,						Three Months Ended   March 31,
	2018			2017			2018			2017
Service cost — benefits earned during the period	$	14,213		$	13,760		$	5,105		$	4,358
Non-service costs (credits):
Interest cost on benefit obligation	31,007			32,701			7,101			7,565
Expected return on plan assets	(45,667		)	(43,710		)	(10,520		)	(13,350		)
Amortization of:
Prior service cost (credit)	—			20			(9,461		)	(9,461		)
Net actuarial loss	7,782			12,489			—			1,454
Net periodic benefit cost (credit)	$	7,335		$	15,260		$	(7,775	)	$	(9,434	)
Portion of cost (credit) charged to expense	$	2,242		$	7,568		$	(5,605	)	$	(4,678	)