Employee and Retiree Benefit Plans (Tables)

6 Months Ended

Jun. 30, 2020

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, 2020 and 2019.

	Three Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)	2020		2019	2020	2019
Service cost	$	2,166	2,062	446	420
Interest cost	5,763		7,100	794	943
Expected return on plan assets	(6,297)		(6,370)	—	—
Amortization of prior service cost (credit)	183		246	—	(97)
Recognized actuarial loss	4,264		3,508	—	—
Net periodic benefit expense	6,079		6,546	1,240	1,266
Other - curtailment	586		—	(1,825)	—
Other - special termination benefits	8,435		—	—	—
Total net periodic benefit expense	$	15,100	6,546	(585)	1,266
	Six Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)	2020		2019	2020	2019
Service cost	$	4,332	4,124	893	840
Interest cost	11,554		14,251	1,588	1,888
Expected return on plan assets	(12,641)		(12,830)	—	—
Amortization of prior service cost (credit)	366		493	—	(195)
Recognized actuarial loss	8,533		7,022	—	—
Net periodic benefit expense	12,144		13,060	2,481	2,533
Other - curtailment	586		—	(1,825)	—
Other - special termination benefits	8,435		—	—	—
Total net periodic benefit expense	$	21,165	13,060	656	2,533