Employee and Retiree Benefit Plans (Tables)

3 Months Ended

Mar. 31, 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 periods ended March 31, 2021 and 2020.

	Three Months Ended March 31,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)	2021		2020	2021	2020
Service cost	$	1,768	2,166	326	447
Interest cost	4,286		5,791	521	794
Expected return on plan assets	(6,133)		(6,344)	—	—
Amortization of prior service cost (credit)	156		183	—	—
Recognized actuarial loss	5,279		4,269	(7)	—
Net periodic benefit expense	$	5,356	6,065	840	1,241