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Share-based payments - Key assumptions of the share option (Details) - Binomial option pricing model
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of terms and conditions of share-based payment arrangement      
Discount rate 17.00% 17.00% 24.00%
Risk-free interest rate 3.00% 4.00% 4.00%
Volatility 46.00% 51.00% 52.00%
Dividend yield 0.00% 0.00% 0.00%