Retirement Plans and Other Postretirement Benefits (Tables)

9 Months Ended

Sep. 30, 2020

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,
	2020		2019		2020		2019		2020		2019		2020		2019
Service cost — benefits earned during the period	$	14,058	$	12,476	$	42,174	$	37,427	$	5,559	$	4,593	$	16,677	$	13,777
Non-service costs (credits):
Interest cost on benefit obligation	29,642		34,211		88,925		102,632		6,464		7,473		19,393		22,420
Expected return on plan assets	(46,861)		(42,971)		(140,582)		(128,913)		(10,019)		(9,603)		(30,057)		(28,809)
Amortization of:
Prior service credit	—		—		—		—		(9,394)		(9,456)		(28,182)		(28,366)
Net actuarial loss	8,653		10,646		25,959		31,938		—		—		—		—
Net periodic benefit cost (credit)	$	5,492	$	14,362	$	16,476	$	43,084	$	(7,390)	$	(6,993)	$	(22,169)	$	(20,978)
Portion of cost (credit) charged to expense	$	736	$	7,593	$	2,349	$	22,837	$	(5,286)	$	(4,966)	$	(15,798)	$	(14,846)