Employee and Retiree Benefit Plans (Tables)

3 Months Ended

Mar. 31, 2018

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)		2018	2017	2018		2017
Service cost	$	2,255	2,032		494		425
Interest cost		6,737	6,719		874		966
Expected return on plan assets		(7,506)	(7,185)		–		–
Amortization of prior service cost (credit)		257	254		(10)		(18)
Recognized actuarial loss		5,215	3,554		–		–
Net periodic benefit expense	$	6,958	5,374		1,358		1,373