Employee and Retiree Benefit Plans (Tables)

6 Months Ended

Jun. 30, 2017

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)		2017	2016	2017		2016
Service cost	$	2,030	2,770		424		675
Interest cost		6,287	8,865		967		1,107
Expected return on plan assets		(6,475)	(9,698)		–		–
Amortization of prior service cost (credit)		254	321		(19)		(20)
Amortization of transitional asset		–	–		–		2
Recognized actuarial loss		3,509	3,718		–		36
Net periodic benefit expense	$	5,605	5,976		1,372		1,800
	Six Months Ended June 30,
	Pension Benefits			Other Postretirement Benefits
(Thousands of dollars)		2017	2016	2017		2016
Service cost	$	4,062	5,923		849		1,348
Interest cost		13,006	14,473		1,933		2,215
Expected return on plan assets		(13,660)	(15,083)		–		–
Amortization of prior service cost (credit)		508	640		(37)		(41)
Amortization of transitional asset		–	–		–		2
Recognized actuarial loss		7,063	7,247		–		75
Curtailments		–	822		–		(19)
Net periodic benefit expense	$	10,979	14,022		2,745		3,580