Retirement Plans and Other Postretirement Benefits (Tables)

9 Months Ended

Sep. 30, 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   September 30,						Nine Months Ended   September 30,						Three Months Ended   September 30,						Nine Months Ended   September 30,
	2018			2017			2018			2017			2018			2017			2018			2017
Service cost — benefits earned during the period	$	14,167		$	13,715		$	42,501		$	41,144		$	5,275		$	4,280		$	15,825		$	12,839
Non-service costs (credits):
Interest cost on benefit obligation	31,172			32,439			93,517			97,316			7,037			7,490			21,111			22,470
Expected return on plan assets	(45,713		)	(43,568		)	(137,140		)	(130,703		)	(10,520		)	(13,350		)	(31,561		)	(40,051		)
Amortization of:
Prior service cost (credit)	—			20			—			61			(9,461		)	(9,461		)	(28,382		)	(28,382		)
Net actuarial loss	8,021			11,975			24,062			35,924			—			1,279			—			3,838
Net periodic benefit cost (credit)	$	7,647		$	14,581		$	22,940		$	43,742		$	(7,669	)	$	(9,762	)	$	(23,007	)	$	(29,286	)
Portion of cost (credit) charged to expense	$	2,524		$	7,231		$	7,535		$	21,692		$	(5,359	)	$	(4,841	)	$	(16,083	)	$	(14,523	)