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Share-based compensation - PSU activity (Details) - 2014 Plan
 6 Months Ended
Jun. 30, 2021
$ / shares
shares
Other disclosure  
Vesting period (in years) 4 years
Performance share units ("PSUs")  
Number of shares  
Non-vested at beginning of period (in shares) | shares 212,614
Vested (in shares) | shares (132,368)
Forfeited (in shares) | shares (2,916)
Non-vested at end of period (in shares) | shares 77,330
Weighted average grant-date fair value  
Non-vested at beginning of period (in dollars per share) | $ / shares $ 42.32
Vested (in dollars per share) | $ / shares 33.09
Forfeited (in dollars per share) | $ / shares 57.56
Non-vested at end of period (in dollars per share) | $ / shares $ 57.56