Retirement Plans and Other Postretirement Benefits (Tables)

9 Months Ended

Sep. 30, 2019

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,
	2019			2018			2019			2018			2019			2018			2019			2018
Service cost — benefits earned during the period	$	12,476		$	14,167		$	37,427		$	42,501		$	4,593		$	5,275		$	13,777		$	15,825
Non-service costs (credits):
Interest cost on benefit obligation	34,211			31,172			102,632			93,517			7,473			7,037			22,420			21,111
Expected return on plan assets	(42,971		)	(45,713		)	(128,913		)	(137,140		)	(9,603		)	(10,520		)	(28,809		)	(31,561		)
Amortization of:
Prior service credit	—			—			—			—			(9,456		)	(9,461		)	(28,366		)	(28,382		)
Net actuarial loss	10,646			8,021			31,938			24,062			—			—			—			—
Net periodic benefit cost (credit)	$	14,362		$	7,647		$	43,084		$	22,940		$	(6,993	)	$	(7,669	)	$	(20,978	)	$	(23,007	)
Portion of cost (credit) charged to expense	$	7,593		$	2,524		$	22,837		$	7,535		$	(4,966	)	$	(5,359	)	$	(14,846	)	$	(16,083	)