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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2017
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 nine months and quarter ended September 30, 2017 and 2016. 
 
 
Quarter Ended  
 September 30,
 
Nine Months Ended 
 September 30,
 
 
2017
 
2016
 
2017
 
2016
 
 
(Thousands of dollars)
Stock-based compensation expense:
 
 
 
 
 
 
Time-based restricted stock units
 
$
582

 
$
631

 
$
1,678

 
$
1,847

Performance-based restricted stock units
 
689

 
823

 
1,396

 
2,502

Total stock-based compensation expense
 
$
1,271

 
$
1,454

 
$
3,074

 
$
4,349