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Share-based compensation | 6 Months Ended
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Jun. 30, 2019
Share-based compensation |
Share-based compensation | 8 Share-based compensation ​ The Company’s share-based compensation plans include the 2014 Amended and Restated Share Option Plan (the “2014 Plan”) and inducement grants under Rule 5653(c)(4) of the NASDAQ Global Select Market with terms similar to the 2014 Plan (together the “2014 Plans”). At the annual general meeting of shareholders in June 2018, the Company’s shareholders approved amendments of the 2014 Plan, increasing the shares authorized for issuance by 3,000,000 to a total of 8,601,471. The Company previously had a 2012 Equity Incentive Plan (“2012 Plan”). As of June 30, 2019, 14,000 fully vested share options are outstanding (December 31, 2018: 32,567) under the 2012 Plan. ​
| a) | 2014 Plan
​ Share-based compensation expense recognized by classification included in the Consolidated statements of operations and comprehensive loss in relation to the 2014 Plan was as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
| ​ | Three months ended June 30, | ​ | Six months ended June 30,
​ |  | 2019 |  | 2018 | ​ | 2019 | ​ | 2018
​ | ​ | (in thousands)
Research and development | ​ | $ | 2,119 | ​ | $ | 993 | ​ | $ | 4,099 | ​ | $ | 1,799
Selling, general and administrative | ​ | ​ | 2,457 | ​ | ​ | 1,212 | ​ | ​ | 4,755 | ​ | ​ | 2,933
Total | ​ | $ | 4,576 | ​ | $ | 2,205 | ​ | $ | 8,854 | ​ | $ | 4,732
​ Share-based compensation expense for the six months ended June 30, 2019 increased by $4.1 million when compared to six months ended June 30, 2018 as a result of the appreciation of the share price and increase in number of grants. Share-based compensation expense for the three months ended June 30, 2019 increased by $2.4 million when compared to three months ended June 30, 2018 as a result of the appreciation of the share price. ​ Share-based compensation expense recognized by award type was as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended June 30, | ​ | Six months ended June 30,
​ |  | 2019 |  | 2018 | ​ | 2019 | ​ | 2018
​ | ​ | (in thousands)
Award type | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Share options | ​ | $ | 2,004 | ​ | $ | 1,096 | ​ | $ | 4,080 | ​ | $ | 2,155
Restricted share units (“RSUs”) | ​ | ​ | 995 | ​ | ​ | 540 | ​ | ​ | 1,925 | ​ | ​ | 1,286
Performance share units (“PSUs”) | ​ | ​ | 1,577 | ​ | ​ | 569 | ​ | ​ | 2,849 | ​ | ​ | 1,291
Total | ​ | $ | 4,576 | ​ | $ | 2,205 | ​ | $ | 8,854 | ​ | $ | 4,732
​ As of June 30, 2019, the unrecognized share-based compensation expense related to unvested awards under the 2014 Plans were: ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Unrecognized |  | Weighted average
​ |  | share-based |  | remaining
​ | ​ | compensation | ​ | period for
​ | ​ | expense | ​ | recognition
Award type | ​ | (in thousands) | ​ | (in years)
Share options | ​ | $ | 21,695 | ​ | 3.08
Restricted share units | ​ | ​ | 6,955 | ​ | 2.10
Performance share units | ​ | ​ | 9,867 | ​ | 2.12
Total | ​ | $ | 38,517 | ​ | 2.66
​ The Company satisfies the exercise of share options and vesting of RSUs and PSUs through newly issued ordinary shares. ​ Share options ​ The following table summarizes option activity for the six months ended June 30, 2019: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options
​ | ​ | Number of | ​ | Weighted average
​ |  | ordinary shares |  | exercise price
​ | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2018 | ​ | 2,673,712 | ​ | $ | 15.09
Granted | ​ | 530,276 | ​ | $ | 36.05
Forfeited | ​ | (41,602) | ​ | $ | 11.88
Expired | ​ | (543) | ​ | $ | 12.26
Exercised | ​ | (218,087) | ​ | $ | 11.45
Outstanding at June 30, 2019 | ​ | 2,943,756 | ​ | $ | 19.16
​ | ​ | ​ | ​ | ​ | ​
Fully vested and exercisable at June 30, 2019 | ​ | 1,264,254 | ​ | $ | 12.37
Outstanding and expected to vest at June 30, 2019 | ​ | 1,679,502 | ​ | $ | 24.27
​ | ​ | ​ | ​ | ​ | ​
Total weighted average grant date fair value of options issued during the period (in $ millions) | ​ | ​ | ​ | $ | 11.2
Proceeds from option sales during the period (in $ millions) | ​ | ​ | ​ | $ | 2.5
​ Share options are priced on the date of grant and, except for certain grants made to non-executive directors, vest over a period of four years, the first 25% vests after one year from the grant date and the remainder vests in equal quarterly installments, over years two, three and four. Any options that vest must be exercised by the tenth anniversary of the grant date. The fair value of each option issued is estimated at the respective grant date using the Hull & White option pricing model with the following weighted-average assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended June 30, | ​ | Six months ended June 30,
Assumptions |  | 2019 | ​ | 2018 | ​ | 2019 | ​ | 2018
Expected volatility | ​ | 75% | ​ | 75% | ​ | 75% | ​ | 75%
Expected terms | ​ | 10 years | ​ | 10 years | ​ | 10 years | ​ | 10 years
Risk free interest rate | ​ | 2.17% - 2.63% | ​ | 2.99% - 3.07% | ​ | 2.17% - 2.87% | ​ | 2.77% - 3.07%
Expected dividend yield | ​ | 0% | ​ | 0% | ​ | 0% | ​ | 0%
​ Restricted share units ​ The following table summarizes the RSUs activity for the six months ended June 30, 2019: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | RSU
​ |  | ​ |  | Weighted average
​ | ​ | Number of | ​ | grant-date fair
​ | ​ | ordinary shares | ​ | value
Non-vested at December 31, 2018 | ​ | 412,321 | ​ | $ | 16.49
Granted | ​ | 145,004 | ​ | $ | 33.62
Vested | ​ | (186,417) | ​ | $ | 12.78
Forfeited | ​ | (14,516) | ​ | $ | 12.90
Non-vested at June 30, 2019 | ​ | 356,392 | ​ | $ | 25.55
​ | ​ | ​ | ​ | ​ | ​
Total weighted average grant date fair value of RSUs granted during the period (in $ millions) | ​ | ​ | ​ | $ | 4.9
​ RSUs vest over one to three years. RSUs granted to non-executive directors will vest one year from the date of grant. ​ Performance share units ​ The following table summarizes the PSUs activity for the six months ended June 30, 2019: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | PSU
​ |  | ​ |  | Weighted average
​ | ​ | Number of | ​ | grant-date fair
​ | ​ | ordinary shares | ​ | value
Non-vested at December 31, 2018 | ​ | 377,169 | ​ | $ | 16.73
Granted | ​ | 132,362 | ​ | $ | 31.71
Vested | ​ | (23,064) | ​ | $ | 5.76
Non-vested at June 30, 2019 | ​ | 486,467 | ​ | $ | 17.80
PSUs awarded but not yet earned | ​ | 96,389 | ​ | $ | 78.15
Total non-vested and discretionary PSUs | ​ | 582,856 | ​ | $ | 27.78
​ | ​ | ​ | ​ | ​ | ​
Total weighted average grant date fair value of PSUs granted and awarded during the period (in $ millions) | ​ | ​ | ​ | $ | 11.7
​ In January 2019, the Company awarded PSUs to its executives and other members of senior management. These PSUs are earned based on the Board’s assessment of the level of achievement of agreed upon performance targets through December 31, 2019. ​
| b) | Employee Share Purchase Plan (“ESPP”)
​ In June 2018 the Company’s shareholders adopted and approved an ESPP allowing the Company to issue up to 150,000 ordinary shares. The ESPP is intended to qualify under Section 423 of the Internal Revenue Code of 1986. Under the ESPP, employees are eligible to purchase ordinary shares through payroll deductions, subject to any plan limitations. The purchase price of the shares on each purchase date is equal to 85% of the lower of the closing market price on the offering date or the closing market price on the purchase date of each three-month offering period. During the six months ended June 30, 2019 4,870 shares were issued. As of June 30, 2019, a total of 142,539 ordinary shares remains available for issuance under the ESPP plan.

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- Definition The entire disclosure for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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