Retirement Plans and Other Postretirement Benefits (Tables)

3 Months Ended

Mar. 31, 2020

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,
	2020			2019			2020			2019
Service cost — benefits earned during the period	$	14,257		$	12,543		$	5,717		$	4,714
Non-service costs (credits):
Interest cost on benefit obligation	29,761			34,352			6,512			7,526
Expected return on plan assets	(46,806		)	(42,893		)	(10,019		)	(9,603		)
Amortization of:
Prior service credit	—			—			(9,394		)	(9,455		)
Net actuarial loss	9,011			11,239			—			—
Net periodic benefit cost (credit)	$	6,223		$	15,241		$	(7,184	)	$	(6,818	)
Portion of cost (credit) charged to expense	$	1,342		$	8,244		$	(5,456	)	$	(4,817	)