Employee and Retiree Benefit Plans (Tables)

3 Months Ended

Mar. 31, 2019

Employee and Retiree Benefit Plans [Abstract]

Components of Net Periodic Benefit Expense

	Three Months Ended March 31,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)		2019	2018	2019		2018
Service cost	$	2,062	2,255		420		494
Interest cost		7,151	6,737		945		874
Expected return on plan assets		(6,460)	(7,506)		–		–
Amortization of prior service cost (credit)		247	257		(98)		(10)
Recognized actuarial loss		3,514	5,215		–		–
Net periodic benefit expense	$	6,514	6,958		1,267		1,358