Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2018

Incentive Plans [Abstract]

Share-Based Plans, Amounts Recognized

	Nine Months Ended
	September 30,
(Thousands of dollars)		2018	2017
Compensation charged against income before tax benefit	$	36,348	28,264
Related income tax benefit recognized in income		5,532	8,695