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Share-based compensation - RSU activity (Details) - The 2014 Plan [Member]
$ / shares in Units, $ in Millions
 3 Months Ended
Mar. 31, 2024
USD ($)
$ / shares
shares
Other disclosure  
Vesting period (in years) 4 years
Non-executive directors  
Other disclosure  
Vesting period (in years) 1 year
Restricted share units ("RSUs")  
Number of shares  
Non-vested at beginning of period (in shares) | shares 2,264,369
Granted (in shares) | shares 1,160,800
Vested (in shares) | shares (659,240)
Forfeited (in shares) | shares (58,401)
Non-vested at end of period (in shares) | shares 2,707,528
Weighted average grant-date fair value  
Non-vested at beginning of period (in dollars per share) | $ / shares $ 18.07
Granted (in dollars per share) | $ / shares 5.59
Vested (in dollars per share) | $ / shares 20.30
Forfeited (in dollars per share) | $ / shares 17.39
Non-vested at end of period (in dollars per share) | $ / shares $ 12.19
Other disclosure  
Total weighted average grant date fair value of RSUs granted during the period (in millions) | $ $ 6.5
Restricted share units ("RSUs") | Non-executive directors  
Other disclosure  
Vesting period (in years) 1 year
Minimum | Restricted share units ("RSUs")  
Other disclosure  
Vesting period (in years) 1 year
Maximum | Restricted share units ("RSUs")  
Other disclosure  
Vesting period (in years) 3 years