Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Share Based Compensation Expense

The table below sets forth the total amount of stock-based compensation expense for the quarters ended March 31, 2019 and 2018.

	Quarter Ended    March 31,
	2019		2018
	(Thousands of dollars)
Stock-based compensation expense:
Time-based restricted stock units	$	840	$	642
Performance-based restricted stock units	475		677
Total stock-based compensation expense	$	1,315	$	1,319