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Stock Options (Tables) | 12 Months Ended
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Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Weighted Average Assumptions Used in Estimating Fair Value of Stock Options |
|  | As of December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
Valuation assumptions: |  |  |  |  |  |  |  |  |  |  |  |
Expected dividend yield |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
Expected volatility |  |  | 37.60 | % |  |  | 39.37 | % |  |  | 41.82 | %
Expected term (years) |  |  | 5.86 |  |  |  | 6.06 |  |  |  | 6.07 |
Risk-free interest rate |  |  | 2.6 | % |  |  | 1.9 | % |  |  | 1.6 | %
Summary of Outstanding Stock Option Activity | Stock option activity during the periods indicated is as follows:
|  | Number of shares |  |  | Weighted average exercise price |  |  | Weighted average remaining contractual term |
Balance at December 31, 2015 |  |  | 756,692 |  |  | $ | 2.10 |  |  |  | 7.6 |
Granted |  |  | 359,319 |  |  |  | 2.60 |  |  |  | 9.4 |
Exercised |  |  | (58,155 | ) |  |  | 2.36 |  |  |  | 3.5 |
Expired/Forfeited |  |  | (17,469 | ) |  |  | 2.13 |  |  |  | 2.6 |
Balance at December 31, 2016 |  |  | 1,040,387 |  |  | $ | 2.25 |  |  |  | 7.8 |
Granted |  |  | 471,144 |  |  |  | 14.72 |  |  |  | 9.1 |
Exercised |  |  | (284,455 | ) |  |  | 2.07 |  |  |  | 5.2 |
Expired/Forfeited |  |  | (23,449 | ) |  |  | 8.22 |  |  |  | — |
Balance at December 31, 2017 |  |  | 1,203,627 |  |  | $ | 7.06 |  |  |  | 8.1 |
Granted |  |  | 1,009,134 |  |  |  | 9.94 |  |  |  | 6.8 |
Exercised |  |  | (341,456 | ) |  |  | 2.24 |  |  |  | 1.3 |
Expired/Forfeited |  |  | (464,256 | ) |  |  | 11.81 |  |  |  | — |
Balance at December 31, 2018 |  |  | 1,407,049 |  |  | $ | 8.73 |  |  |  | 8.1 |
Vested and exercisable at December 31, 2018 |  |  | 504,287 |  |  | $ | 6.28 |  |  |  | 7.0 |
Vested and expected to vest at December 31, 2018 |  |  | 1,407,049 |  |  | $ | 8.73 |  |  |  | 8.1 |

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: 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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