Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2018

Incentive Plans [Abstract]

Share-Based Plans, Amounts Recognized

	Three Months Ended
	March 31,
(Thousands of dollars)		2018	2017
Compensation charged against income before tax benefit	$	7,549	8,148
Related income tax benefit recognized in income		894	2,529