Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2022

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 June 30,				Six Months Ended June 30,				Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021		2022		2021		2022		2021
Service cost — benefits earned during the period	$	13,406	$	14,939	$	27,737	$	30,618	$	4,017	$	4,341	$	8,235	$	8,898
Non-service costs (credits):
Interest cost on benefit obligation	26,723		24,614		53,746		49,283		4,283		4,095		8,746		8,257
Expected return on plan assets	(46,494)		(50,706)		(92,888)		(101,314)		(11,511)		(10,361)		(23,021)		(20,722)
Amortization of:
Prior service credit	—		—		—		—		(9,447)		(9,427)		(18,894)		(18,854)
Net actuarial loss (gain)	3,989		3,989		8,757		7,974		(3,436)		(2,641)		(6,418)		(5,046)
Net periodic benefit	$	(2,376)	$	(7,164)	$	(2,648)	$	(13,439)	$	(16,094)	$	(13,993)	$	(31,352)	$	(27,467)
Portion of benefit charged to expense	$	(4,722)	$	(8,614)	$	(8,012)	$	(16,625)	$	(11,523)	$	(9,608)	$	(22,418)	$	(19,136)