Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2021

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,
	2021		2020		2021		2020		2021		2020		2021		2020
Service cost — benefits earned during the period	$	14,939	$	13,859	$	30,618	$	28,116	$	4,341	$	5,401	$	8,898	$	11,118
Non-service costs (credits):
Interest cost on benefit obligation	24,614		29,522		49,283		59,283		4,095		6,417		8,257		12,929
Expected return on plan assets	(50,706)		(46,915)		(101,314)		(93,721)		(10,361)		(10,019)		(20,722)		(20,038)
Amortization of:
Prior service credit	—		—		—		—		(9,427)		(9,394)		(18,854)		(18,788)
Net actuarial loss (gain)	3,989		8,295		7,974		17,306		(2,641)		—		(5,046)		—
Net periodic benefit cost/(benefit)	$	(7,164)	$	4,761	$	(13,439)	$	10,984	$	(13,993)	$	(7,595)	$	(27,467)	$	(14,779)
Portion of cost/(benefit) charged to expense	$	(8,614)	$	271	$	(16,625)	$	1,613	$	(9,608)	$	(5,056)	$	(19,136)	$	(10,512)