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Share-based compensation - Options (Details) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 $ / shares shares | Sep. 30, 2017 $ / shares shares | Sep. 30, 2018 $ / shares shares | Sep. 30, 2018 £ / shares shares | Sep. 30, 2017 $ / shares shares
Number of options granted (in shares) | shares | 757,273 | 8,756,211 | 12,187,614 | 12,187,614 | 28,959,363
Weighted average fair value (in dollars per share) | $ / shares | $ 1.09 | $ 0.34 | $ 0.82 |  | $ 0.34
RSU |  |  |  |  |
Number of options granted (in shares) | shares | 1,259,760 |  | 7,966,716 | 7,966,716 |
Weighted average fair value (in dollars per share) | $ / shares | $ 1.67 |  | $ 1.40 |  |
Exercise price | £ / shares |  |  |  | £ 0.001 |
Vesting period (in years) |  |  | 4 years | 4 years |
Contractual term (in years) |  |  | 10 years | 10 years |
First anniversary | RSU |  |  |  |  |
Vesting percentage (as a percent) |  |  | 25.00% | 25.00% |
Annual installments over the following three years | RSU |  |  |  |  |
Vesting percentage (as a percent) |  |  | 75.00% | 75.00% |

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + 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)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which option holders acquired shares when converting their stock options into shares. + References No definition available. + Details
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- Definition Percentage of vesting of share-based compensation awards. + 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 (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (a)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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