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Share-based compensation - RSU activity (Details) - 2014 Plan - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 9 Months Ended
Mar. 31, 2017
Sep. 30, 2021
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)   467,344
Granted (in shares)   547,271
Vested (in shares)   (214,368)
Forfeited (in shares)   (102,697)
Non-vested at end of period (in shares)   697,550
Weighted average grant-date fair value    
Non-vested at beginning of period (in dollars per share)   $ 43.56
Granted (in dollars per share)   36.48
Vested (in dollars per share)   40.48
Forfeited (in dollars per share)   40.94
Non-vested at end of period (in dollars per share)   $ 39.34
Other disclosure    
Total weighted average grant date fair value of RSUs granted during the period (in millions)   $ 20.0
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