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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2018
Summary of Stock-Based Compensation Expense | The following table summarizes the Company’s stock-based compensation expense (in thousands):
|  | Year Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
Time-based restricted stock and time-based restricted stock units |  | $ | 2,687 |  |  | $ | 1,992 |  |  | $ | 2,281 |
Performance-based restricted stock and performance-based restricted stock units |  |  | 3,144 |  |  |  | 2,805 |  |  |  | 2,210 |
Stock appreciation rights |  |  | 370 |  |  |  | 251 |  |  |  | 184 |
Employee stock purchase plan |  |  | 143 |  |  |  | 139 |  |  |  | 113 |
Total stock-based compensation |  | $ | 6,344 |  |  | $ | 5,187 |  |  | $ | 4,788 |
Summary of Assumptions Used to Estimate Fair Value of Each SAR | For SARs issued in the years ended December 31, 2018 and December 31, 2017, respectively, the assumptions shown in the following table were used:
|  | December 31, |
|  | 2018 |  |  | 2017 |
Dividend yield |  |  | 0 | % |  |  | 0 | %
Average risk-free interest rate |  |  | 2.7 | % |  |  | 2.0 | %
Expected term (years) |  |  | 5 |  |  |  | 5 |
Expected volatility |  |  | 40.5 | % |  |  | 42.3 | %
SAR Activity | SAR activity under the Plan and all predecessor stock incentive plans is as follows:
|  | SARs |  |  | Weighted-Average Grant Price Per Share |  |  | Weighted- Average Remaining Contractual Life (Years) |  |  | Aggregate Intrinsic Value as of December 31, 2018 |
Outstanding at December 31, 2015 |  |  | 524,154 |  |  | $ | 6.57 |  |  |  |  |  |  |  |  |
Granted |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Exercised |  |  | (248,704 | ) |  | $ | 5.55 |  |  |  |  |  |  |  |  |
Canceled |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2016 |  |  | 275,450 |  |  | $ | 9.79 |  |  |  |  |  |  |  |  |
Granted |  |  | 37,478 |  |  | $ | 35.38 |  |  |  |  |  |  |  |  |
Exercised |  |  | (34,812 | ) |  | $ | 8.07 |  |  |  |  |  |  |  |  |
Canceled |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2017 |  |  | 278,116 |  |  | $ | 13.45 |  |  |  |  |  |  |  |  |
Granted |  |  | 21,260 |  |  | $ | 56.59 |  |  |  |  |  |  |  |  |
Exercised |  |  | (60,900 | ) |  | $ | 5.27 |  |  |  |  |  |  |  |  |
Canceled |  |  | — |  |  | $ | — |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2018 |  |  | 238,476 |  |  | $ | 19.26 |  |  |  | 5.3 |  |  | $ | 9,562,603 |
Vested at December 31, 2018 |  |  | 192,232 |  |  | $ | 13.08 |  |  |  | 4.5 |  |  | $ | 8,896,335 |
Exercisable at December 31, 2018 |  |  | 192,232 |  |  | $ | 13.08 |  |  |  | 4.5 |  |  | $ | 8,896,335 |
Time-Based Restricted Stock and Time-Based Restricted Stock Units [Member] |
Restricted Stock Activity | Time-based restricted stock and restricted stock unit activity under the Plan and all predecessor stock incentive plans is as follows:
|  | Time-based Restricted Stock and Restricted Stock Unit |  |  | Weighted-Average Grant Price Per Share |
Nonvested at December 31, 2015 |  |  | 215,814 |  |  | $ | 14.72 |
Granted |  |  | 115,748 |  |  | $ | 18.82 |
Vested |  |  | (87,696 | ) |  | $ | 21.17 |
Forfeited |  |  | (266 | ) |  | $ | 21.95 |
Nonvested at December 31, 2016 |  |  | 243,600 |  |  | $ | 15.80 |
Granted |  |  | 72,402 |  |  | $ | 36.27 |
Vested |  |  | (162,372 | ) |  | $ | 14.45 |
Forfeited |  |  | (512 | ) |  | $ | 18.68 |
Nonvested at December 31, 2017 |  |  | 153,118 |  |  | $ | 26.90 |
Granted |  |  | 87,264 |  |  | $ | 54.72 |
Vested |  |  | (84,550 | ) |  | $ | 26.65 |
Forfeited |  |  | (284 | ) |  | $ | 35.05 |
Nonvested at December 31, 2018 |  |  | 155,548 |  |  | $ | 42.68 |
Performance-Based Restricted Stock and Performance-Based Restricted Stock Units [Member] |
Restricted Stock Activity | Performance-based restricted stock activity under the Plan is as follows:
|  | Performance-based Restricted Stock and Performance-based Restricted Stock Units |  |  | Weighted-Average Grant Price Per Share |
Nonvested at December 31, 2015 |  |  | 58,194 |  |  | $ | 19.69 |
Granted |  |  | 89,850 |  |  | $ | 17.92 |
Vested |  |  | (29,898 | ) |  | $ | 17.86 |
Forfeited |  |  | (1,314 | ) |  | $ | 16.85 |
Nonvested at December 31, 2016 |  |  | 116,832 |  |  | $ | 18.32 |
Granted |  |  | 86,614 |  |  | $ | 28.77 |
Vested |  |  | (86,788 | ) |  | $ | 18.64 |
Forfeited |  |  | — |  |  | $ | — |
Nonvested at December 31, 2017 |  |  | 116,658 |  |  | $ | 25.85 |
Granted |  |  | 80,570 |  |  | $ | 35.26 |
Vested |  |  | (106,022 | ) |  | $ | 23.52 |
Forfeited |  |  | — |  |  | $ | — |
Nonvested at December 31, 2018 |  |  | 91,206 |  |  | $ | 36.86 |

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- Definition Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for stock appreciation rights awards that were outstanding at the beginning and end of the year, and the number of stock appreciation rights awards that were granted, exercised or converted, forfeited, and expired during the year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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