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Share-based compensation - Summary of changes in share based compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Restricted stock units    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 193,939 207,641
Granted (in shares) 127,598 77,679
Vested (in shares) (78,988) (77,719)
Forfeited (in shares) (6,358) (4,160)
Outstanding, end of period (in shares) 236,191 203,441
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 40.89 $ 35.36
Granted (in dollars per share) 33.98 48.11
Vested (in dollars per share) 38.51 34.19
Forfeited (in dollars per share) 42.20 35.81
Outstanding, end of period (in dollars per share) $ 37.91 $ 40.67
Total weighted-average grant-date fair value $ 4.3 $ 3.7
LTIP linked to TRS    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 89,222 96,402
Granted (in shares) 44,210 24,630
Vested (in shares) (32,355) (29,409)
Forfeited (in shares) (1,024) (1,007)
Outstanding, end of period (in shares) 100,053 90,616
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 42.10 $ 39.62
Granted (in dollars per share) 41.12 48.62
Vested (in dollars per share) 38.20 39.51
Forfeited (in dollars per share) 45.89 41.72
Outstanding, end of period (in dollars per share) $ 42.89 $ 42.08
Total weighted-average grant-date fair value $ 1.8 $ 1.2
LTIP awards linked to other performance conditions    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 220,715 403,768
Granted (in shares) 176,844 98,522
Vested (in shares) (43,155) (135,804)
Increase above target (cancelled) (in shares) (14,604) (26,111)
Forfeited (in shares) (4,098) (4,031)
Outstanding, end of period (in shares) 335,702 336,344
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 41.03 $ 35.15
Granted (in dollars per share) 33.98 48.10
Vested (in dollars per share) 34.12 33.48
Increase above target (in dollars per share) 43.90 34.13
Forfeited (in dollars per share) 44.36 39.67
Outstanding, end of period (in dollars per share) $ 38.04 $ 39.64
Total weighted-average grant-date fair value $ 6.0 $ 4.7