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Share-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |
Summary of Share Option Activity | Share option activity under the 2014 Plan is summarized as follows:
|  | Number of Shares |  |  | Weighted- Average Exercise Price |  |  | Weighted-Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands)(1) |
Outstanding as of January 1, 2018 |  |  | 3,767,130 |  |  | $ | 12.69 |  |  |  |  |  |  |  |  |
Granted |  |  | 830,420 |  |  | 44.77 |  |  |  |  |  |  |  |  |
Exercised |  |  | (507,632 | ) |  | 10.07 |  |  |  |  |  |  |  |  |
Forfeited or cancelled |  |  | (73,328 | ) |  | 19.93 |  |  |  |  |  |  |  |  |
Outstanding as of December 31, 2018 |  |  | 4,016,590 |  |  | $ | 19.47 |  |  |  | 7.24 |  |  | $ | 93,520 |
Options exercisable as of December 31, 2018 |  |  | 2,451,584 |  |  | $ | 10.11 |  |  |  | 6.51 |  |  | $ | 78,280 |
Options unvested as of December 31, 2018 |  |  | 1,565,006 |  |  | $ | 34.14 |  |  |  | 8.39 |  |  | $ | 15,240 |
(1) | The aggregate intrinsic value of options is calculated as the difference between the exercise price of the share options and the fair value of the Company’s ordinary shares for those share options that had exercise prices lower than the fair value of the ordinary shares as of the end of the period.
Summary of RSU Activity | RSU activity for the year ended December 31, 2018 is summarized as follows:
|  | RSUs |  |  | Average Grant Date Fair Value (in dollars per share) |
Outstanding as of January 1, 2018 |  |  | 154,459 |  |  |  | 29.05 |
Granted |  |  | 313,747 |  |  |  | 40.17 |
Vested |  |  | (38,594 | ) |  |  | 26.30 |
Forfeited |  |  | (21,091 | ) |  |  | 37.27 |
RSUs Outstanding at December 31, 2018 |  |  | 408,521 |  |  | $ | 37.16 |
Summary of Share-based Compensation Expense Classified in Consolidated Statements of Operations and Comprehensive Loss | Share-based compensation expense for the years ended December 31, 2018, 2017 and 2016 is classified as operating expenses in the consolidated statements of operations and comprehensive loss as follows:
|  | For the Year Ended December 31, |
|  | 2018 |  |  | 2017 |  |  | 2016 |
|  | (in thousands) |
Research and development expenses |  | $ | 9,172 |  |  | $ | 7,670 |  |  | $ | 4,936 |
General and administrative expenses |  |  | 6,424 |  |  |  | 4,473 |  |  |  | 1,911 |
Total share-based compensation expense |  | $ | 15,596 |  |  | $ | 12,143 |  |  | $ | 6,847 |
Employee Stock Option [Member] |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |
Summary of Fair Value of Share Options Granted | The assumptions used in the Black-Scholes option pricing model to determine the fair value of share options granted to employees during the period were as follows:
|  | For the Year Ended December 31,
|  | 2018 |  | 2017 |  | 2016
Risk-free interest rate |  | 2.49% – 3.01% |  | 1.49% – 2.23% |  | 1.15% – 2.18%
Expected term (in years) |  | 3.00 – 6.25 |  | 3.00 – 6.25 |  | 3.00 – 6.25
Expected volatility |  | 64.17% – 70.90% |  | 68.95% – 72.24% |  | 60.89% – 68.76%
Expected dividend yield |  | 0% |  | 0% |  | 0%

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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited 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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