Incentive Plans (Tables)

6 Months Ended

Jun. 30, 2018

Incentive Plans [Abstract]

Share-Based Plans, Amounts Recognized

	Six Months Ended
	June 30,
(Thousands of dollars)		2018	2017
Compensation charged against income before tax benefit	$	18,970	16,722
Related income tax benefit recognized in income		2,463	5,425