Employee and Retiree Benefit Plans (Tables)

6 Months Ended

Jun. 30, 2021

Retirement Benefits [Abstract]

Components of Net Periodic Benefit Expense

The table that follows provides the components of net periodic benefit expense for the three-month and six-month periods ended June 30, 2021 and 2020.

	Three Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)	2021		2020	2021	2020
Service cost	$	1,768	2,166	327	446
Interest cost	4,300		5,763	521	794
Expected return on plan assets	(6,155)		(6,297)	—	—
Amortization of prior service cost (credit)	156		183	—	—
Recognized actuarial loss	5,281		4,264	(8)	—
Net periodic benefit expense	5,350		6,079	840	1,240
Other - curtailment	—		586	—	(1,825)
Other - special termination benefits	—		8,435	—	—
Total net periodic benefit expense	$	5,350	15,100	840	(585)
	Six Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)	2021		2020	2021	2020
Service cost	$	3,536	4,332	653	893
Interest cost	8,586		11,554	1,042	1,588
Expected return on plan assets	(12,288)		(12,641)	—	—
Amortization of prior service cost (credit)	312		366	—	—
Recognized actuarial loss	10,560		8,533	(15)	—
Net periodic benefit expense	$	10,706	12,144	1,680	2,481
Other - curtailment	—		586	—	(1,825)
Other - special termination benefits	—		8,435	—	—
Total net periodic benefit expense	$	10,706	21,165	1,680	656