| Business combinations and disposal Revitalised (Details) - Revitalised $ in Millions | Jul. 18, 2017  USD ($) | 
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| Business Acquisition [Line Items] | |
| Share capital acquired (as a percent) | 100.00% | 
| Cash consideration | $ 2.7 | 
| Contingent consideration arrangements, range of outcomes, value, high | $ 1.4 | 
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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. No definition available. 
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- Definition Percentage of voting equity interests acquired at the acquisition date in the business combination. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
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