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Share-based payment arrangements - Stock option valuation (Details) - LTIP - $ / shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Share-based payment arrangements    
Fair value at grant date $ 14.77 $ 4.62
Share price at grant date 13.38 8.40
Exercise price $ 14.46 $ 9.02
Expected volatility (as a percent) 76.51% 93.22%
Expected life 6 years 1 month 6 days 5 years 10 months 24 days
Risk-free interest rate (based on government bonds) 1.00% 1.20%