Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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 six months ended June 30, 2018 and 2017. 

	Quarter Ended    June 30,				Six Months Ended   June 30,
	2018		2017		2018		2017
	(Thousands of dollars)
Stock-based compensation expense:
Time-based restricted stock units	$	435	$	556	$	1,077	$	1,111
Performance-based restricted stock units	378		705		1,055		705
Total stock-based compensation expense	$	813	$	1,261	$	2,132	$	1,816