| Share-based compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands | 3 Months Ended | |
|---|---|---|
| Mar. 31, 2018 | Mar. 31, 2017 | |
| Total share-based compensation expense included in the consolidated statements of operations | ||
| Total share-based compensation expense | $ 4,672 | $ 2,686 | 
| Research and development | ||
| Total share-based compensation expense included in the consolidated statements of operations | ||
| Total share-based compensation expense | 2,554 | 1,359 | 
| General and administrative | ||
| Total share-based compensation expense included in the consolidated statements of operations | ||
| Total share-based compensation expense | $ 2,118 | $ 1,327 | 
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- References No definition available. 
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