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Share-based compensation - Employee Share Purchase Plan - Narrative (Details) - ESPP - Employee Stock [Member] - shares
1 Months Ended 3 Months Ended
Jun. 30, 2018
Mar. 31, 2019
Share-based compensation    
Ordinary shares available for issue 150,000 144,283
Discounted rate for purchase of shares 85.00%  
Number of shares issued   3,126