Employee and Retiree Benefit Plans (Tables)

6 Months Ended

Jun. 30, 2018

Employee and Retiree Benefit Plans [Abstract]

Components of Net Periodic Benefit Expense

	Three Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)		2018	2017	2018		2017
Service cost	$	2,254	2,030		493		424
Interest cost		6,707	6,287		874		967
Expected return on plan assets		(7,453)	(6,475)		–		–
Amortization of prior service cost (credit)		256	254		(9)		(19)
Recognized actuarial loss		5,181	3,509		–		–
Net periodic benefit expense	$	6,945	5,605		1,358		1,372
	Six Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)		2018	2017	2018		2017
Service cost	$	4,509	4,062		987		849
Interest cost		13,444	13,006		1,748		1,933
Expected return on plan assets		(14,959)	(13,660)		–		–
Amortization of prior service cost (credit)		513	508		(19)		(37)
Recognized actuarial loss		10,396	7,063		–		–
Net periodic benefit expense	$	13,903	10,979		2,716		2,745