Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

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 quarter and nine months ended September 30, 2018 and 2017. 

	Quarter Ended    September 30,				Nine Months Ended   September 30,
	2018		2017		2018		2017
	(Thousands of dollars)
Stock-based compensation expense:
Time-based restricted stock units	$	1,085	$	582	$	2,162	$	1,678
Performance-based restricted stock units	1,033		689		2,088		1,396
Total stock-based compensation expense	$	2,118	$	1,271	$	4,250	$	3,074