Employee and Retiree Benefit Plans (Tables)

3 Months Ended

Mar. 31, 2017

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)		2017	2016	2017		2016
Service cost	$	2,032	3,153		425		673
Interest cost		6,719	5,608		966		1,108
Expected return on plan assets		(7,185)	(5,385)		–		–
Amortization of prior service cost (credit)		254	319		(18)		(21)
Recognized actuarial loss		3,554	3,529		–		39
Curtailments		–	822		–		(19)
Net periodic benefit expense	$	5,374	8,046		1,373		1,780