| Share-based compensation - Options Granted to Nonemployees (Details) - Nonemployees - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
| Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015 | |
| Share based compensation | ||||
| Number of options granted outstanding (in shares) | 3,074,600 | 3,074,600 | ||
| Share based payment charge (benefit) | $ 274 | $ 1,707 | $ (108) | $ 2,506 | 
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- Definition Represents the expense (benefit) recognized during the period arising from equity-based compensation arrangements with nonemployees. No definition available. 
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- References No definition available. 
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- Definition Number of options outstanding, including both vested and non-vested options. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
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