v3.7.0.1

Stock-Based Compensation (Tables) | 6 Months Ended
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Jun. 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 six months and quarter ended June 30, 2017 and 2016.
|  | Quarter Ended June 30, |  | Six Months Ended June 30,
|  | 2017 |  | 2016 |  | 2017 |  | 2016
|  | (Thousands of dollars)
Stock-based compensation expense: |  |  |  |  |  |
Time-based restricted stock units |  | $ | 556 |  |  | $ | 597 |  |  | $ | 1,111 |  |  | $ | 1,216 |
Performance-based restricted stock units |  | 705 |  |  | 809 |  |  | 705 |  |  | 1,680 |
Total stock-based compensation expense |  | $ | 1,261 |  |  | $ | 1,406 |  |  | $ | 1,816 |  |  | $ | 2,896 |

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